One possibility is that, if you get in earlier, as the company grows you're more likely to be promoted to more senior positions, which might be good for professional development. Relatedly, you're more likely to work on a larger variety of things since there isn't enough manpower to let you specialize, which can help develop a larger skillset.

Whether they balance unpaid overtime/stress, however, can be iffy. If you can get in without the bad parts, though, it can be good (but that's less start-up and more small company).