X11 is a networked windowing system. Your client and your server might be on different machines. GLX packages up OpenGL commands into network packets, spits them across the X11 network pipe, then unpacks them at the other end. This lets you run accelerated 3D remotely: the client could be a simulation on a mainframe, the display could be your desktop machine in your office. GLX does a number of other X11 related things that couldn't be packaged into OpenGL.