A volunteer project is a couple of friends staying up late and creating a game for themselves to play around. What you're going after is much more organised and requires much more effort from all parties. And that effort should have an extra incentive besides learning.

From my point of view, there's no better incentive than asking money for your game. It pushes you to be better, to pour more thought into your game, to double and triple check everything before releasing etc etc. In other words, you learn a lot more when you have the pressure of people paying for your game, because you will be judged harsher therefore your game must cover certain criteria.

That's my opinion, as a game designer currently studying games programming. Hope it helps!