It’s a “whites of their eyes” reference. The enemy is decked out in full plate so lucky shots are all that would strike true at that range. Their hope was that the archers’ aim would improve, and therefore enemy casualties, once they scooted in a smidge. However, that underlies the overall issue of inexperience from bottom to top. Or it was just for cinematic effect, I’ve never watched the director’s commentary.
Considering they seem to have unlimited arrows, they should fire upon them for lucky shots, and then continue with greater accuracy as the orcs close the distance.
There’s also a “shock and awe” component. At that range, you’re just dealing with a bunch of random shots. Imagine instead getting up closer, and meeting a solid wall of arrows, destroying the whole front of your army!
Maybe Orcs have different psychology but that would discourage/panic most sentient beings
