Ain't you ever played a video game? This is like an item shop where you can buy equipment to give you a power-up, or a potion to restore your HP, or an alternate costume.

[Okay, so Ango hasn't actually figured out the buying part yet since there's no money in the Dream Sphere, but he's not a filthy capitalist pig so that's fine.]

If you mean "why" in the philosophical sense, it's because I'm a Saver who's got fuck all when it comes to attack power, so I figure I can help more teams this way.