Well some people might want a new colored case for the complete alt they bought in the past, so they already have the Diffuser. But if you go with buying the alt in parts you need the diffuser.
It has been my observation that if a case is high profile, it says so. If it is low profile, it just says case. See the title of this page? No high profile. Look at image number 12 - definitely low profile. Look at any top-view image - pretty flat and smooth, eh? Look at any similar image from a high profile page. See the difference?