If you battle with your pokemon, you increase your friendship with that pokemon. It wouldn't make sense to have a feature like that if it didn't change anything, but if I ever knew what bonus you got when gaining your pokemons' trust, I've forgotten. What bonuses are gained when you gain the trust of your pokemon, by battling and training them?
If you are talking about just friendship then yes @Studoku's answer is correct but if you meant the new mechanic in X and Y called Affection, that is a little different.
- Resistant to having stats lowered
- Evasiveness increases
- Similar effect to Endure where stays at 1 hp
- Pokemon levels faster
- More critical hits( says something like ‘Pokemon Meowth is in sync with Trainer Name’)
All of these are permanent when there Affection is full as well, I do believe.
There are several effects of having a high friendship.
- Certain pokemon such as Togepi are evolved by raising their friendship.
- The move Return, which can be taught to anything which can learn TMs has power based on the friendship stat of the user. Its power reaches 102 at max friendship, often making it the strongest Normal move a pokemon can learn. Frustation, the opposite of Return, has power based on how low the user's friendship is.
- As of Gen VI, an extremely high friendship has other effects in battle. The full effects and chances haven't been fully discovered yet but the effects of extremely loyal pokemon include.
- Surviving moves that would otherwise KO
- Recovering from Status Effects
- Increased Critical chance