For me it's both. There's the writing practice I get
https://steemit.com/success/@plotbot2015/every-post-is-practice
and there's the exposure to my other, paid writings.
http://www.intergalacticmedicineshow.com/cgi-bin/mag.cgi?do=columns&vol=randall_hayes&article=_index

But there's also the social aspect. I put a lot of stuff out there on various platforms, and have for years now, but Steemit is the first place people have written back and tried to engage at any level, really. The crypto-currency is just a little bonus hit of dopamine. I haven't even tried to figure out the cash-out options yet, although seeing the little numbers go up is definitely a form of reinforcement.