What does college (in the absence of much other work experience) actually signal?

• You either can afford it or have the background and educational achievements to win significant financial aid. (More expensive colleges do better at this.)
• You have the independence to live away from your parents, care for yourself in an unfamiliar environment, and not do anything that would scare HR over several years.
• You have the security at home to commit to doing something for several years in a row; e.g. you don't have an ailing parent or grandparent you need to care for, and you're not likely to have to leave the workforce to care for an infant or two.
• You have the commitment to stay focused on one broad project for years at a time and the diligence to study for and pass tests.
• You can learn material similar to that required for the job and/or are willing to do pointless work in order to please employers. Maybe you even learned something relevant!
• You personally have a baseline level of literacy and mathematical competence. High school education in the US won't guarantee it; our standardized test scores don't look good on the whole.

None of these signals are strong differentiators, but most of them look like things that long-term employers genuinely want regardless of the field you end up in. After all, if college isn't that useful and on-the-job training lasts a while, these traits make a candidate more likely to stick around and be useful afterwards, and if college is necessary, they're still helpful.

Most of these things are also signaled by a few years of suitable work experience; it's just hard to get that when companies expect degrees even for the few entry-level positions available, because they'd rather see these things up front than put their cash on the line to let you prove them.