Well there's a simple answer to it, on some servers, zombie especially, people are joining the game at the start of a new map and so they need to know when does it start because it takes some time to join it and check it for yourself, I know it doesn't take that much time but when you do this more often it can get irritating. The reason for this is that it is not worth joining a zombie server in half game etc. because in most cases you wouldn't get to play as a Hunter on the current map.

Hope it makes it a bit clearer to understand why people like to have a time left timer in their hostname.