Militia can serve a use in battle as well, just by sheer numbers. Having an army in the province boosts loyalty but it does not matter what quality the units are, peasants count as much as knights in terms of boosting loyalty.
Militia are good for garrison duties in safer provinces. Militia type units are most common in the early part of the early era, when most factions are bulking up their first armies and gradually they are replaced by stronger and more powerful units as the years go by. Most are lightly armed and armored and serve well as support units to more professional soldiers which make up the core of the army. The fighting quality of militia type units varies from average to terrible, depending on the units.
They are cheap to train, have lower upkeep, and have low requirements to build. Examples include peasants, slav warriors, and urban militia. Militias are cheap, conscripted soldiers which historically have made up the bulk of medieval armies.
