It is just something for people to do once the storyline is over. Plus, how everyone plays is different than competitive battling, which is a start. It can also be something for want to battle since back then in the Advance games there were no Wi-Fi compatible systems or games. Really, all it is something to do during your free time, and is somewhere you can battle if you don't have any friends to battle with, or don't feel like going over to their house just to battle. Anyways, you don't gain something by doing it, just a symbol. Yes, but do you gain anything from playing a game? No, all it is a video game, they only put it in there so people won't be bored after finishing the storyline, and have something to do other than "catch 'em all" or train Pokemon with good Nature and IVs, but no where to battle with them except the Battle Tower, Battle Frontier (or if you are willing to find a friend to battle against). Elite Four is really the place to go if you want EXP gain, BT or BF is just to test movesets, or just have fun battling.