So I always thought that was to have the server show up even if no one is playing there ~
Would be great if TF 2 servers had it more often, because like 90% of the time it's 23/24 with one spot kept for the replay, which I actually like to have, but it's annoying these aren't filtered by "servers that aren't full", because they have an unaccessable slot telling you they're full when they're at 23/24.