Supplies units can be seen as worthless in many situations. This is because in areas where you controls cities or harbors, these structures are sufficient to resplesnish all units without using up the supply unit.
Not so in some severe conditions (advancing in hostiles regions without any control on nearby cities, in winter, mountain, etc.)
Now there is also 3 important additional roles for these slow wagons, and you will see them differently I bet
1. A supply unit with at least some ammo points remaining give a +10% offensive/defensive fire bonus to all units in battle.
2. A supply unit with at least some general supply points will prevent your troops from surrendering in case of siege.
3. A supply unit can absorb the hit points taken from bad weather by negating 1 hit point damage with an extra 5 supplies points usage (no limit on that, it can dry up your wagon!). Note that the message list the theorical hit points damages, without taking into account this special "supply shielding rule". You will see the difference in your army though