So that when people see the talkgroup ID pop up on their scanner, they know who it is, even though they can't monitor it.
It also helps somebody when looking for a certain agency. Say you have a state wide trunking system, and the local police are on it but they're encrypted. You go looking for them in the database, find them, see that they're encrypted and that's that. If they weren't listed, you would look for them in the database and not know where they are.