Also having attended college and actually successfully passed its knowledge tests and graduated proves that you have both the discipline and mental capability for certain jobs.

I'm in software development and have been part of the process of hiring people and from the point of view of an employer, for a candidate to an entry level position that college diploma is an indicator that the person in question has the knowledge and capabilities to do that kind of job.

Mind you, in my area fortunatelly there are other ways to indicate that - for example, having participated in Open Source projects or, even better, having your own Open Source project with actual users that you've had to support (which in my view can put somebody above somebody else who merelly has a college diploma) - though that's generally only for smaller companies since large ones will have HR filter candidates before the ever reach the actual domain experts and HR can't judge skill like that and instead will go for "formal stamp of approval" shit such as college diplomas.

That said, the college diploma stops being important after junior level, unless it's one from a handful of very prestigious institutions and even then it won't work on domain experts, only non-expert manager types - if a company is hiring people for mid and above expertise levels based on which college they've attended, that place is going to be a political shithole of incompetence better avoided by those who aren't skilled at or interested in progressing their career through social games.