you can get more experience points for your games, review tactics, find where you can get free items. say for example, exp game, you can search for your games there and request for experience points. Gaming is all about tactics. That's how you become a better gamer.

What I have understood of their Pre-registration is that, there are games in that app that aren't available yet on stores. And if you Pre-register, you could a beta tester for these games. By rewards, if you mean Vouchers, then it is since they said they give game codes, which are literally translated to Game vouchers. 

Haven't tried downloading a game through their application, but I bet it will redirect you to the game store. Although if it is for beta testing, you might be redirected to where you can download the APK