Well, keeping Pokemon inside is kinda of what they do. But y' can put one Pokemon in another Pokemon's Pokeball, so I can't put Snivy in Charizard's Pokeball. Pokeballs also know when a Pokemon is wild or not and if th' Pokemon already has a trainer it won't work on 'em. That's why it's super important for your Pokemon to have a Pokeball even if they don't ever go inside it. You can also transfer Pokeballs using a Pokeball transfer machine. It really helps when ya have more than six Pokemon 'cause you can't have more than six of your Pokemon with ya at the same time. If ya catch a seventh th' pokeball actually stops workin' until you transfer one to a Professor or somethin'.