What they just said is only the tip of the iceberg, or however the hell that saying goes. With more indepht training, you can have more creative movesets (like Eletrivire with Ice Punch), pokes with better stats: on top of the 63 you get from EV training, you can also get more via IV breeding, 31 to be exact. You can have more powerful teams, such as sweepers, walls, etc. Basically, pokes with specific roles. And the most important, if you wanna battle thru wi-fi, you really must EV train in order to survive. If ya need help, just ask.