OpenGL has a client/server model, which stills apply event if all happens on the same machine.
The server is the OpenGL implementation (the driver), and the client is the application.
So when you compile your display list, commands results can be cached in server-side memory space, that is anything from vanilla system memory, to AGP memory or even onboard video memory.
When the client calls the display list, the server can use previously cached results without having to copy anything (from client to server).
That’s one benefit of display lists. The other is that during display list compilation, the server has the opportunity to optimize the calls being made in the display list to make the execution faster.