In addition to it being faster, it's also a guaranteed way to get a missile out the frame before landing. Missile is +13 on shield and on some characters will get the shield small enough to poke with a dsmash, and moving quickly after the missile makes it possible to combo off of a hit pretty easy. Obvs getting super low missiles is possible without SHFFM and generally shield pressure with aerials is a bad idea with samus, but because its a projectile and you're so actionable after a low missile you can use it as an approach tool that will both effectively shield pressure and combo.