Many of the bosses in the game are simply stronger monsters. They're not meant to add to the plot or anything of that sort: they're simply like the rulers of their particular area of the wilderness. Do you want an explanation for every malborro or wolf you see? You shouldn't, because in a fantasy setting you come to expect to see monsters. The bosses in FFXII aren't significant (except in cases where you're fighting the Empire, other people, etc., obviously) because they're simply more powerful creatures residing in a part of the world you're traveling through. If they were more than that, they would certainly warrant some sort of explanation. But they're not.