Depending on what phenomena a set of equations describe, if the equations break down at some point it becomes of interest to figure out the reason. The situatio

Depending on what phenomena a set of equations describe, if the equations break down at some point it becomes of interest to figure out the reason. The situation cannot be ignored because it hints that the phenomena may not be properly understood. The equations of general relativity do not hold on the quantum scale, and those of quantum mechanics cannot be used when gravity is large. This is a break down. The bridge between the two could be unifying the two "worlds" or have a damn good reason why not. Nature has a good motivator for unifying called black-holes. Inside black-holes you have large gravity and also quantum scale stuff that has been eaten or squished together or just tiny space. It would be rather difficult to solve the black-hole problem starting from scratch, as black-holes arise out of general relativity equations. So you start from quantum scale and cross-check with the other end or vice versa. If you are weary of the word "unify" from a philosophical perspective, then it may help to realize that the word unify comes to apply because we (humanity) have discovered and built up more knowledge from imperfect bundles. Suppose in 1906 (before quantum mechanics or general relativity) Einstein had come up with some universal theory/model that inherently worked everywhere, it probably would never have been termed as a unification theory.