One, practical: I'm European, and found that my options for living in other countries were narrowed by not having a degree. I would probably have moved to Japan for several years if I had had one, since it's a prerequisite for getting a work visa in that country and (obviously) being a westerner in that country causes you to stick out like a sore thumb and they're super-bureaucratic about this. I'm just personally interested in Japan and Japanese culture, so I regret foreclosing that option for myself.
Two, socioeconomic: although a bit of an introvert, I think there's enormous value to the personal networks you can build up in college, in terms of meeting potential mentors, colleagues and so forth. Now, I think there's (pan-)social downsides to that system, and somewhat disapprove of the whole 'Greek system.' But overall I think college provides access to considerable social capital, whose utility may not be apparent in the short term but which makes a huge difference for most people over the longer term. Most of us are not Bill Gates, and the fact is that Gates inherited a lot of social capital from his parents.
Three, intellectual: Although I enjoy being a self-directed learner and feel pretty confident about my smarts, I do go through deep and horrible abysses of self-doubt, and often feel ashamed of not having fulfilled my intellectual potential by completing and exploiting a degree. I'm just shy of 43 now; whereas in my 20s I felt very proud of my ability to learn, navigate life, and make a living sans degree (and still do in many respects; I've built an impressive resume as a high-level film technician over the last decade despite never having taken a film class), these days I hate admitting that I don't have a degree and feel that my career overall has stalled for lack of direction and focus. True, people often tout their degree as a substitute for actual skill and experience, and an excess focus on credentialism is a negative aspect of our society and economy. But at the same time, I am sick to the back teeth of explaining why I don't have a degree, having to sell myself from scratch every time instead of simply saying 'oh yes, I have a BS in Economics from Berkeley' or whatever. Also, I struggle with a nagging feeling of being a dilettante and lacking a solci intellectual focus, so when I'm down in the dumps I don't feel like a smart guy who has a decent life in defiance of the odds, but a bullshit artist whose run of relative good luck could end at any time. Having to regularly re-sell yourself to yourself on your own merits is just as tiresome as having to sell yourself to others. It's a large and omnipresent confidence drain.
