Likely for match shooting to quickly engage multiple targets by better controlling the muzzle as demonstrated by Jerry Miculek below at 5:20 minute:
When I run fast point shooting defensive drills on multiple targets, I wrap my hand around the handguard with thumb extended so I can "point" with my thumb which extends the support hand almost straight out and Jerry demonstrates at 12:20 minute:
And also demonstrated by US Army AMU engaging multiple targets and grip covered in detail starting at 1:30 minute:
But when I want to stabilize the front sight of my AR for longer range/precision shots, I bring my support hand on the handguard closer to the receiver so as to rest elbow against the ribcage and this stabilizes front sight/crosshairs of the scope.