It's what you want it to be. Dating area, social network building place, source for information and tutors, source of certification, funnel to corpo jobs, etc...

Acculturation to bourgeois and haut bourgeois norms depending on the college, for elite colleges a way for the rich and those highly likely to be professionally successful to associate to the benefit of both, a way to launder IQ tests (the SAT and ACT), a four year holiday and networking opportunity, an entree into better jobs, a way to get the public to subsidise research they don’t understand. College has many purposes, just like school. They share the sorting and ranking function but for college there childcare one is much attenuated.

Adult daycare? An expensive way to delay the onset of adulthood a few more years? An elaborate proxy for an otherwise-prohibited IQ test?

Mostly it's a piece of paper. For many professions, no matter how pointless the journey, that's a very necessary piece of paper.