Supply wagons are your primary source of healing abilities and buffs. Cheap, but highly useful. Therefore, it's encouraged to have at least one supply wagon in two or three parties to keep your units recuperated throughout consecutive battles.
While it's possible to have supply wagons attack opponents and assume offensive stance, it's not recommended to do so due to its lacking mobility, speed, durability, and offensive capabilities. As previously stated, supply wagons' main role in battles are to provide support to more offensively-capable ally units. Keep them far and protected from the enemies' reach.