but dont you use the same move? so whats the difference, how can you get shield grabbed by A but not the c-stick?
Say you sh and fair with A. It's not just A you have to hit but FORWARD on the Control Stick. This will cause you to move (however slightly you hit forward A and hold back to correct the DI) to move slightly forward. This can especially in pressure situations magnify to big spacing errors.
With the C-stick if you shorthop, the only button you have to hit to do a fair is left or right on the c-stick depending on your character orientation. Since you don't need to use the C-stick at all you will not move your character from his aerial position. In fact, for all aerials except nair (which can't be done with the C-stick) you can actually hold any direction and still perform the desired aerial with the C-stick.
So, therefore, the C-stick greatly enhances your aerial mobility and decreases mistakes while performing aerial attacks. Only downside is that it takes effort and practice to get fingers fast enough to use the C-stick effectively.