Akin to snipers. They are an important part of the military but not the main force.
More like light artillery. To snipe with a bow, even a war bow, takes years of training.
But, plinking a load of arrows onto a bunch of enemies who don’t carry good shields will mess up their formation — making an infantry charge effective.
Armies in that era were more into slings. Cheaper to make and carry, and just as effective if your helmet is not steel.