Every side quest you do in Coldharbour has the potential to add extra fighters and/or support for the final assault. For each side quest you don't do, a certain character or group of characters will not appear, making that part of the final assault that they would help in harder.
To ensure you have the full complement of extra characters, you need to complete all the side quests around Coldharbour and turn all the quest hub icons in the western and eastern planes to white. (You can't currently access those in the north, as they are beyond the locked gate which opens after the War Council.)