Thats a loaded question............................literally.
The reason for differant actions are for differant uses. Fast actions, generally speaking, tend to throw lighter presentations better than heavy or slower actions.
Lighter, also called faster actions tend to be for the lumber tossers, AKA, plugs, while the slow/heavy tend to generate better casts with heavy baits. The slow actions also act as sort of shock absorber for the bait throwers as well.
The main differance is in the way they load up for the cast. Fast actions tend to snap the cast, while the slow actions tend to lob the cast.