I think it was David Locke who was a proponent of players declaring for the draft if they think they'll get picked and want to play professionally.

His explanation is that in college there are limits as to how much interaction a player can have with school coaches, but in the NBA there are no such limits. A player can get all kinds of practice and coaching in without the NCAA looking over their shoulder.

I'm not sure I agree 100%, but with the G-league and opportunities for young players to be drafted and then play rather than sitting on an NBA bench, I can see some merit to the argument.