Maps contain :
The real layout of the lines (metros & funiculars), tracks, ...
The position of the stations and their platforms.
Closed stations or never opened.
Tracks and tunnels of service and lines connection.
Extension projects of the network.
Tracks number, open dates of the sections
Should be very useful for writers etc. since the stylised maps usually distort distance and layout to make the routes clearer.
There are also maps for Los Angeles and Montreal.
Note to mods - we seem to lack a Belgium tag...
later - as pointed out in a comment from fiddlingfrog, there's also an official TFL map of the London lines which is geographical rather than the stylised version, and shows things around the lines such as parks.