The purpose of the flight of arrows was to blanket the area ahead of an army advancing to meet the front line of your army. It was mathematically, the only way to have a chance at killing a large number of targets, as being accurate at that range was nearly impossible, even for the best longbowman.

So they went with numbers over precision. This also allowed them to effectively slow down the pace of the advancing frontline if every time they loose their arrows, they all have to stop and take cover.

It has a tertiary goal of likely killing any wounded still on the ground. It’s not a technique you’d use when your forces have fully engaged as it’d kill just as many of your units as the enemy’s.

Finally, it could be used in strategic encounters to force the enemy to stagger/get out of position if you could cause just one part of their advancing line to stop, it’d essentially break or weaken the enemy’s frontline.