Stripping of the Membranes

When getting induced, is getting your membranes stripped a mandatory part of the process or can you choose to not have it done? What exactly would be the reason you would have to have it done? I have my paperwork showing the different methods of induction, but they don't tell WHY those methods would be used. I already know about Prostaglandins, Breaking the Water, and the cervical ripening balloon, but I'm not so sure about Membrane Stripping or what its for.