Ear cushions can be understand as a part of eyeglasses frames that can bring people comfortable wearing. Usually, long time wearing eyeglasses frames may result in different degrees of irritation. Eyeglasses with ear cushions can decrease that "rubbing" and discomfort. These cushions, also called pads, can be adjusted by yourself to fit for your face and ears. Anyway, ear cushions are good for people wearing eyeglasses.
Maybe. But I don't think it is necessary to wear ear cushions together with eyeglasses. As long as your glasses fit you properly, that is to say, they are neither too tight nor too loose, they won't cause pain on you. Once you get pain from wearing your eyeglasses, it means that they are too tight for you. In this case, you need to have them adjusted in order to fit you exactly. Or you can take them back to the place you bought them and ask the staff there to help adjust your glasses.