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! Jessie: No! Ash: Finish it off with Thunder! Seviper faints Ash: Yay!
So yeah, it is supposed to make it look more realistic. Would Pikachu sweep Meowth in one shot with Thunderbolt? Don't think so. [1]: http://Pokemondb.net/pokebase/93512/in-the-anime-why-do-they-let-you-attack-twice
Cuz its a real-time cartoon based on a turn based RPG. it obviously wouldn't be very exciting to watch if Ash just sat there and let Pikachu eat a DynamicPunch while he stood idly by until it was his turn, now would it? and lets face it: the anime (at least the US version) is primarily geared towards children who aren't gonna overanalyze it. the games require much more strategy and capacity for tactical reasoning than watching a children's cartoon. don't read so much into it.
