On Intense, Geara's Clefable has a seed that will activate in the sky field, setting 8 turns of Tailwind for the duo. However, on Normal/"Reborn", (at least according to the wiki, I've never played through Normal myself) it simply has a Sitrus Berry. Therefore it's pretty much straight advantageous to change it to a sky field, as it makes a number of their mons lose their neutrality to Rock (rock moves are Rock/Fire on the volcano, so Gyarados, Typhlosion, Dragonite etc. don't care about them). Mantine with Bounce is a good setter and also comes with Wide Guard to protect from Rock Slide, Erup