Because at a time, they will run out of arrows.
I explain all the tactic :
First launch your poor troops, then when enemy is weak, launch your elite troops.
In game meaning : use bad armors till defender run out of ammo, then you could use better armors to finnish the job without being hurt by ranged.
Otherwise you will lose good armors in first part of siege as they will be full of arrows (anyway your troops will die)