When you interact with him, you gain money (you get more money for more extreme actions). You can then use this money to buy new weapons and other methods of interaction. You can also buy new skins (so your buddy looks different) or new game modes (like Low Gravity, for example).
Imagine you have a buddy that you can do pretty much anything to even blowing him up with grenades or even throwing baseballs at him.
Tomb Raider: Legend on the GBA delivers a solid platforming experience that actually manages to mix in a healthy dose of the gunplay and puzzles that the console versions contained.