You do not need to delete the archived Cards in general. Trello provides enough capacity for holding the archived Cards. However, you may want to delete them if you want to make sure they are not restored.
Archive Manager allows you to delete the archived Cards altogether.
You can enable Archive Manager as follows:
Menu -> Power-Ups -> RedSync Settings -> Edit Power-Up settings
At the bottom of the Setting window, there are two option switches.
Turn on the Maintenance switch, then RedSync shows the Archive Manager board menu.
