Cheap units used to impede a more expensive enemy unit's movement by forcing the unit to charge them or waste a turn manoeuvring around them. They are usually positioned so that the pursuit/overrun direction of the enemy unit will waste even more of its time and potentially set up counter charges by allied units. This applies particularly as a strong tactic against deathstars and units with frenzy. Good chaff units have to be expendable so are always cheap, however other useful qualities include them being flying, skirmishers and/or having a high Movement value which allow them to get into position more easily.
6,141 times viewed