I’m behind again this morning so not sure if anyone has already addressed this. But it seems reasonable that LE would note their location to 1) prove their existence (which proves that rounds were fired - particularly important when there is a report of shots fired but no one was hit); and 2) have some frame of reference for how close or how far away the casings were from the reported scene of the shooting, which can help prove if those casings are pertinent to the incident being investigated.
And it is not out of the realm of possibility for those casings to be found where they were. When we are shooting at the range, it is crazy how casings fly around. Even though I may be standing in one spot and fairly motionless as I address the target, one casing may fly downward, while the next one may fly completely over my head, and the next one may fly some distance sideways. And if I am doing rapid fire, it increases the chances of there being a fairly wide spread in where the casings end up, primarily due to recoil of the gun (and yes, I am working on trying to better control that).
If LE were to investigate my performance at the range, they would likely find a plethora of casings clustered somewhere in front of my feet as well as to the sides and behind me. (One of my shooting instructors once described casing trajectories as “chaos theory in action.”)