There's a lot of self-selection, and the classes and extracurricular resources are therefore allowed to be geared towards smarter students, and that's nice. You'll also get more opportunities to learn about current research in your chosen field, which improves your grad school chances.

A lot of the value is if you plan to get a job straight out of college, going to a top n school will have a name brand advantage (not without reason).

However, controlling for smartness and research experience, I think that where you did your undergrad doesn't matter all that much for grad school.