They do it to make it look more powerful, or to finish a Pokemon off. The writers of the Anime will do things in the Anime that you wouldn't in the main game, to keep things interesting. EXAMPLE:
Ash: Pikachu, use Thunderbolt!
Ash: Finish it off with Thunder!
So yeah, it is supposed to make it look more realistic. Would Pikachu sweep Meowth in one shot with Thunderbolt? Don't think so.