The option to have more sprites under certain conditions. Example, sprites for a third speed. However, the default characters do not have these sprites in them, it's just for customizability when creating new characters. So you could use them like:
Swim sprites that are different to fly (and interchange if you surface, like S3)
Extra running sprites (that are set to walkanim, runanim and sprintanim) [And just to note, you could in theory have true walking frames using that, rather than the kind of jog that the default characters have]
Ability sprites (like slowfall would activate a different sprite)
And so on and so forth. It's nothing spectacular and should probably be considered later on, but it just adds to the customizability, which, in my opinion, is what makes SRB2.