When you whip the butter you are incorporating air so the finished product is lighter in texture. The butter will add moisture to your skin whether its whipped or not, but a lot of people like the lighter texture. Also, when you whip the butter you can add other ingredients that contribute something - like tapioca powder can be added to take away some of the greasy feel of the butter. It's really all about textures. I have a thing (probably some OCD) about greasy stuff on my hands. I can't stand liquid lotion on my hands. I'll apply it to my body but then immediately head to the sink to wash it off my hands. I don't have that same icky feeling with whipped body butters that have something added to make them less greasy. Same for lotion bars - they're very moisturizing but the beeswax in them adds a texture that doesn't make me cringe.