If you have the maintenance mode enabled for an agent, this agent is no longer considered when checking the replication status. That means it still delivers content, but you don't see pending replication (e.g. in Classic UI's siteadmin) even if this queue is full, but the others are not.
it is mostly used if the authors should not care about the queue size of an agent, e.g. because you are performing a time-consuming maintenance on a publish system, which is not reachable from enduser (and when thus a growing replication queue doesn't matter until the system is being put back into service).