Historically speaking, the conservation of energy has been deduced from the quantitive physical theories from the Renaissance onwards. Gradually the importance of this law was recognised and eventually placed at the pinnacle of physical science. It was the quantitative nature that allowed for the empirical discovery that something was being conserved. This is the empirical & scientific view.
But when we look at energy as a substance, that is something that remains conserved through accidental change, we see in fact that the conservation of energy is a law that characterises the meaning and the substance of energy. This is a philosophical view - we have here an invention of a concept.
Certainly, I feel the first paragraph is correct. I'm unsure of the second. It seems very likely that this must be true. But I've never seen this argument made before. Has this indeed been done (or disputed) by someone?
Given that there are several conservation laws in physics - angular momentum, linear momentum etc. One needs to modify the above observation; and simply say are there substances in physics? Is there only one substance or many substances. If there are many then one cannot convert (part of) one to another as that would violate its identity as a substance.
Historically, the conservation of mass is the first discovered and probably dates from antiquity although I have no plausible reference for it.
The angle I'm trying to aim for is that conservation is important from 'purely' philosophical aims in the idea of substance; in the same way that indivisibility is important for the idea of atoms which does date from antiquity (and is paid homage to in Newtons Principia).