tre5
+1y
The purpose? If it was my choice MacPhersons would never be used in cars. They don't ride as good with adjustable suspension (hydro or air) as a double wishbone does. As far as the actual purpose, it's the same as anything... it's the cars suspension. I'm not really sure what your asking though.
The grease on the powerballs is just what they come with. It's not nearly enough to do the job, probably just to keep it from being metal to metal until they are actually used. I use wheel bearing grease on them and always try to make the fitting accessible because they will need grease often to keep the noise to a minimum.