[JKTrix]

One of the 'benefits' of getting the unown is that you can get Seals for your pokeball. There's a boy in one of the cities who will give you a set of seals with the letter of the unown you have at the head of your party.

Seals, if you didn't know, allow you to customize your pokeball's opening animation. You can buy bubbles and feathers and fire and music notes and all other kinds of stuff, but the only way you can get letters to spell things is by getting the unown.