I think that's a main difference. With a degree you tend to learn / get taught things upfront of which you don't know whether you'd ever need them, and you may not. You kind of trust, or hope, that what they teach you has some relevance and is useful.
Many subjects from courses I took during my degree I'd never have picked up "on the side" during my jobs that came after. But I can actually also confidently say that each and every course I took left some impressions/concepts in my head that later became useful in one way or the other. So for each course I took, had I not taken it, I'd be missing something. Of course there is opportunity cost: could I have used that time in some better way, i.e., even more efficiently? Not sure. There is also the question about the courses that I did not take because, well, there is only so much space you can cram courses into within 6 years :) but maybe there are diminishing returns as well, since many things tend to overlap.