I use it to help with dex completion, I've got 717/720 on my Pokémon X through catching, raising, breeding, trading and transferring.
Other than that I like to use it to have otherwise unobtainable Pokémon in my team on a new play through. For example, on my current AS team I've got Sawsbuck, Tyrantrum and Empoleon because I find the Hoenn pokedex quite boring.