It's just a gimmick Pokemon.
Unown has literally no use, yeah. It's just there for it's massive amount of formrs - Pokemon isn't ALL about battling. But Unown has 28 forms, one for each of the alphabet, and another two for ! and ? characters. Unown forms can be fun to collect. (Like Vivilion's millions of forms) That's really the only reason Unown exist, for a novelty activity in Pokemon.
Also, Unown characters are used as the main alphabet in the Pokemon world, which is a great way to distinguish the real world from the Pokemon world.
Hope I helped. :)