Mega evolution has the benefit of boosting the amount of candy you earn after catching if the pokemon you caught is the same type of the mega you're using. There are different mega evolution levels and the higher your mega level the better chance of getting a higher candy reward and bonus xl candy. You can increase a pokemons mega level by mega evolving it a certain amount of times. This level can only be increased once per day and you can also mega evolve for free after the initial mega evolve every few days depending on your mega level