If I wanted to pose a character and only want just one part of arm or leg to increase size, I could use a slider 
instead of having to go back to edit mesh sculpt to do it. It would save a lot of time. 
If I want to morph one arm or leg, you can't do it in Morphs. You have to morph both limbs.
Facial expressions let you use Left or Right parts of mouth and eye.