In units of 5 rats with a packmaster. They are used to gain tactical advantages, like if i'm about to charge your plague furnace for example. You only get to swing the wrecking ball in your turn, so it is a massive disadvantage if I charge your monks. You can position a rat pack in front of my unit to block my charge into the furnace, allowing me to kill it and overrun into the monk unit to ensure that the combat is fought in your turn. In advanced usage, the principle remains the same but you can change the angle that the blocking unit is facing to gain further tactical advantages.