Why do archer soldiers in the Medieval period shoot arrows into the air to eliminate enemies during a battle?

I don't think it was the preferred mode of Archery in battlefields.

From study of the Shastras and certain contemporary novels and plays, it can be observed that the preferred kill-shot, or at least the one most archers were recommended to practice constantly, was to fire a calftooth or leafblade arrow at charging enemies, preferably at eye level, with a 80–90 kg drawweight bow.

Note that neither are “armour-penetrating” arrows.

High Trajectory shots were discouraged since they presumably had a tendency to “bounce” off Helmets, typically thickest at the top. Even Armour-penetrating Finehead arrows were more likely to get stuck in Armour than carry through and pierce the flesh. They might have been used but the technique itself is kind of treated as a fluke for games and such.