One ongoing ontological problem is that we see things as having or not having a purpose or meaning. Our general feeling is that the purpose or meaning is an additional thing, different from the object, but integral with it, too. The words on the page are words, in some way, and not just black marks. We skip the details of reference and see their purpose directly (they come ready-to-hand).
This extra 'thing' seems to come in and go out of existence in ways that 'real' things do not, and particular it can go partially out of existence. Tools break, we notice the text is in Cyrillic, the keys stick. This presents a case study in tracing a different sort of existence that the basic, physical one, which is still not too complex to actually deal with, and is an opportunity to get clearer data on our basic notions of existence and attention.
It also leads us to analyze out the purpose and meaning, which are separate from our more basic being and look at what that more basic being might be in isolation. What does it mean to just be, without continual attachment to future purpose or accumulated meaning? To what degree are we actually capable of looking at the part of a person, especially ourselves, that is actually a 'thing', without our perspective being invaded by intellectual attachments. (This has interesting applications to why we are so put off by schizophrenia and autism, but it requires way too much background to discuss.)
This perspective has uses of its own.
Unreadiness-to-hand is terminology for this additional layer of existential attachments to an object which we see as having a role. Simply by giving the concept of use some 'meat', this concept improves approaches to design.
Collecting insights into and theories of how things maintain or find readiness-to-hand is a useful way of thinking about how to limit complexity and clarify continuity in interface design, and keep oneself from getting overly invested in mere appearances or attached to idiosyncratic assignments of meaning through sheer logical convenience.
It captures a way of anchoring learning theory and attention to the design, and separates the central issues from those of sheer meaning which obsess programmers, and those of sheer approachability and engagement that obsess artists.