"Supply Troops are not very good at fighting, but can carry resources to your other cities or friends, or carry loot back when you plunder enemy cities."
Level 1 Barracks
Level 1 Fletching
Where to use
Supply Troops are useful for taking out traps in enemy cities or wilderness, so that you don't lose excessive might due to farming or crest hunting. However, they are much less effective than Militiamen.
These units are meat shields, hence other than taking out traps and assorted wall defenses they have no worthy of mention use.