The idea is that Shaymin gets a total change of personality when switching between forms. As the Ground Forme, Shaymin is meek, quiet, and timid. As the Sky Forme, Shaymin is loud, energetic, and outgoing. And personally, I find that these two modes convey Shaymin's personality perfectly. As the Ground Forme, Shaymin resembles a hedgehog, meaning she can roll up into a ball, effectively hiding herself from the world. Her small eyes and stubby legs also give her a squishy plush look, the sort of physique you'd expect to see from an animal that prefers peace and quiet. N contrast, Shaymin's Sky Forme has long legs and a well-defined head, meaning she can move aroud places easily. She has big, inquisitive eyes and big, long ears that look like waving hands, as if to grab attention from those passing by. She also has a petal-like thing hanging from her side that resembles a handkerchief tied around her neck, which traditionally indicates bravery and toughness (for example, cowboys, expeditioners, and some soldiers in rough climates).
And me, I'd get packs when a set is new because I don't care what I want to get; I just want to get something. Buying individual cards is for when you already have a specific idea on what to do with the cards in this set.