>>22202865Some megas (Most notably Garchomp) have lower speed than their megas, so you get one more turn of a faster Pokémon.
Several megas have useful abilities in their regular forms, for example, with Manectric you may want to volt-switch out without mega evolving so that later on you can predict an electric attack and get a lightningrod boost on the switch.
Salamence has a similar thing; if you're going to be switching it out soon anyway, you may as well keep your intimidate for the next time, rather than having an ability that doesn't benefit from switching.
