Use of #nav id on primary and secondary css rules

Why does genesis_SUBTHEME's page.css use #nav on all the primary-links and secondary-links rules? It seems like it's overspecifying them, for no good reason. It's easy to reassign the menus to different blocks on the blocks page, but then the rules as is are useless. Granted that it's easy enough to delete it, and that you're going to be editing the rules anyway for it to matter, but it's still annoying. I know that there are some situations where you need an extra specification to make the cascading rules work, but that doesn't seem to be the case here.

For that matter, I suppose I could ask why bother with the #nav div at all, instead of just using an ordinary region. But mostly I'm curious if there's a method to the css madness.