People also do it so they don't accidentally open something that's got the camera enabled. If you're a school teacher answering questions for students in the evening you don't want to click the wrong thing and turn on video, or close the lid with something running then open it later, having forgotten, and now you're broadcasting. Stuff like that. Whether teh haxx0rs can get at the camera is irrelevant for that reason of using covers.

Personally I wish all my devices had hardware toggles for camera and mic, both. Phone included, since it's only a "phone" a small fraction of the time these days, the rest of the time it's a small Web-stuff device.