Wouldn't archers let loose more arrows thereby getting more kills by firing at will? I'm sure that during battles archers had no shortage of arrows. Was is for the psychological factor of seeing a wall of arrows and losing will to fight? Or was there an actual tactical advantage?
hi! hopefully one of the military experts will be by to help you out but meanwhile there have been a few posts on this tactic, I've found a few to get you started
Did those showers of arrows really exist? - responses from /u/MI13, /u/AsiaExpert, /u/Valkine
What were the military advantages and disadvantages of the Welsh/English Longbow? - /u/MI13 comments on volleys wrt the effectiveness of the longbow
The idea of firing on command does several things. Most importantly, it covers the largest surface area when fired at the same time. More arrows cover more ground at the same time. So you're more effective at hitting groups of people. Second, it allows for control of your indirect fire. You will be able to lead your ground forces up under the cover of heavy fire. Third, it controls your logistics. You control how many arrows your archers have left, which might be needed later.
All that said, the were times when archers were allowed to fire at will. It's just not as effective.