True, but I always had problems to know in what they are useful? Maybe to automate distant connection configuration? Most users only use them as a replacement of getty to start an xsession, I think, and for that use, I wonder if it is really useful.Display managers, and particularly window managers and desktop environments <http://en.wikipedia.org/wiki/Desktop_environment> are often confounded. A window manager is a component of a desktop environment. Decent desktop environments allow you to use the window manager of your choice.
Of course, I respect the fact people use it, but if someone could explain me why, I would be happy to learn.