Because server inevitably will crash. It's like the life and death cycle. Some servers are exceptionally healthy and will keep a clean sheet for a very long time. Some servers crash frequently. EU2 crashes every day, once or twice, in a phenomenon called 'EU2 daily crash'. Every server rises from it's ashes, like a phoenix, some will take longer to reboot than others.
One reason is because of maps. On terror train, the rag dolls constantly move creating lots of polygons (? I think) and it will cause strain on the server and eventually crash it. @Opalium The other reasons for regular maps is things like throwing props as the server has to load the trajectory of every moving object, and a lot of things move in gmod.
