At 150 yards, why would a sniper need a range finder?
Probably because in his wildest imagination he never would have foreseen security so lax that he could get that close.
He probably expected to have been shooting from 300+ yards or more.
It could be he was planning for a head shot and needed the range information for a proper aim. A 5.56 does not shoot in a straight line of sight, it actually arcs upwards then downwards. Knowing the elevation the bullet will hit at a certain distance improves accuracy.
For example if a 5.56 weapon with a 16 inch barrel is zeroed in for a dead center hit at 50 yards, it will likely hit 2.75 inches above the line of sight at 100 yards. A 50 yard zero is best for shots 200 yards or less. Reaching out any farther a 200 yard zero is supposed to be best. Where to zero the weapon is my opinion. The arced flight of the projectile is fact.