Metaphysics seems to be a field that is almost purely abstract. However, I am also interested in knowing how metaphysics has impacted the real world, if at all.
See Hawley's paper Applied Metaphysics for a recent survey. She gives three application areas outside of philosophy: applied ontology in computer science and biology, social ontology, and metaphysics of natural kind terms in psychiatry and medicine. Modal metaphysics of natural kinds is also prominent in the causal theory of reference in linguistics developed by Kripke and Putnam. One more area is high energy physics, where various grand unification projects require careful inspection of metaphysical aspects of their hypothetical fundamental ontologies, see e.g. On the Emergence of Time in Quantum Gravity by Isham and Butterfield (section 4).
The first international workshop on applied ontology was held in 1993, Applied Ontology journal was founded in 2005, and Journal of Social Ontology in 2014, both are backed by international societies. Note that the use of terms "metaphysics" and "ontology" in applied contexts is controversial, some scientists wish to draw a sharp distinction between applied and philosophical meaning of the words, others embrace their unity. E.g. Smith and Klagges write "Applied ontology is a branch of applied philosophy using philosophical ideas and methods from ontology in order to contribute to a more adequate presentation of the results of scientific research". Regardless of linguistic preferences there is one common feature the two share, a prominent role of metaphysical speculation in constructing suitable frameworks for a domain of discourse, some call it "ontological engineering". Another shared function is serving as an incubator of new scientific theories, which generates hypotheses that can then be empirically tested.
Guarino and Musen explain that "linguists and philosophers now work hand-in-hand with traditional computer scientists to build complex information systems with explicit, examinable conceptual models of the environments in which they are intended to operate, of the organizations in which they will be used, and of the data and knowledge that they will process". Even existentialist ontologies of Heidegger and Merleau-Ponty found unexpected applications in AI research, see Dreyfus's Why Heideggerian AI Failed and how Fixing it would Require making it more Heideggerian. Gene ontologies embed gene products into a web populated by biological processes, cellular components and molecular functions in a way that is not species specific, and so it does not reflect the usual ontology.
The natural kinds feature for instance in metaphysical ontologies of diseases or mental disorders, this is ethically sensitive and potentially controversial:"If we take the example of chemical elements as paradigmatic, we may think of natural kinds as determining the intrinsic, essential features of the world, and as drawing sharp, immutable boundaries between different kinds of people". Hawley's own characterization of social ontology is as follows:"Social ontologists study the nature of social reality, for example social groups, institutions, markets, rules, collective act ions, and a myriad other social phenomena. What is the relationship between a group’s action and structure, and the individual actions of group members? What is it for individuals to act jointly? What is it for an institution to structure behaviour?"
Everything we do has metaphysical origins. What we call physical or natural science has failed to establish any mechanism of consciousness, choice, or conscience, and yet these are self-evident. Everyone experiences them, but physics does not explain them.
The motive power behind every discovery, every breath, and every action is a forward-looking, metaphysical expectation and reliance on the realization of things not yet realized. Imagination, desire, volition, movement, and life itself are all witnesses of the metaphysical; while they obey natural laws, they are neither described fully nor circumscribed by any known set of purely physical laws. Some would call this phenomenon faith: the expectation of things not (yet) seen. Many others might not have a name for it. Some would describe it as strictly rational, but I am not aware of any rational defense for this argument. This pattern of self-motive action to materialize hitherto unseen outcomes is common to all life.
Whatever your belief on the subject, belief itself is a further manifestation of the metaphysical, inclining you to act in certain ways in preference to others. In short, it would be difficult or impossible to name anything that has not in some way been affected by the metaphysical.
Conifold's answer is very thorough, but I believe it answers only one of the two questions you have asked. The first question and the one to which Conifold responds is:
What are some real-life applications of metaphysics?
The second, more interesting and thus far neglected question is:
How has metaphysics impacted the real-world, if at all?
The difference between these two questions becomes clear if we consider another example. An enumeration of the 'real-world applications' of music might include prenatal music therapy, whereas the impact of music on the world is so much broader than any single 'application' that it would indeed be difficult to understate.
The mathematician and philosopher C.S. Peirce has this to say about the impact of metaphysics on the world and the hope he has for his own:
To erect a philosophical edifice that shall outlast the vicissitudes of time, my care must be, not so much to set each brick with nicest accuracy, as to lay the foundations deep and massive. Aristotle builded upon a few deliberately chosen concepts - such as matter and form, act and power -- very broad, and in their outlines vague and rough, but solid, unshakable, and not easily undermined; and thence it has come to pass that Aristotelianism is babbeld in every nursery, that "English Common Sense", for example, is thoroughly peripatetic, and that ordinary men live so completely within the house of the Stagyrite that whatever they see out of the windows appears to them comprehensible and metaphysical. Long it has been only too manifest that, fondly habituated though we are to it, the old structure will not do for modern needs; and accordingly, under Descartes, Hobbes, Kant, and others, repairs, alterations and partial demolitions have been carried on for the last three centuries. [...] The undertaking of this present work [of metaphysics] is to outline a theory so comprehensive that, for a long time to come, the entire work of human reason, in philosophy of every school and kind, in mathematics, in psychology, in physical science, in history, in sociology, and in whatever department there may be, shall appear as the filling up of its details. (A Guess at the Riddle, first published in Collected Papers, MS 909)
Professional software developer here, and I can offer you an example from my own experience. The reigning software development paradigm since about 1980 has been "object-oriented programming". The idea being that one uses code to describe a type (or "class"), its states, and possible behaviors, then creates an instance ("object") of that class for actual use. Classes can also be extended to create sub-classes (e.g.,
Square is a sub-class of
Rectangle, with additional behavioral constraints).
In my opinion, one thing that often separates a good programmer from a great programmer is the ability to reason about ontology in this environment. Adding behavior to classes that should not ontologically contain that behavior makes software very difficult to manage as a system grows in size and complexity. This often leads to interesting metaphysical discussions, for example: "What do we really have? A
deleted, or do we have a new thing, a
In my experience, virtually all books on object-oriented programming begin with a discussion of the relevant metaphysical concepts, without ever mentioning (knowing?) that we're discussing philosophy and not programming. But of course, without the philosophical underpinnings it would be impossible to begin programming in any object-oriented language.
There are a number of different senses of metaphysics; one that is often neglected but which stands in the early formation of Western Philosophy is an untitled volume of notes by Aristotle which was named Metaphysics by an editor of his works, to suggest that it be read after another volume of his works titled Physics.
It's in this book that Aristotle discursively and theoretically orders the achievements of Greek science (though they would not have used this word); it's a book that had quite an impact on the later renaissance of the sciences in Western Europe.