Going to negative on the ground run as you slow reduces
the angle of attack and stops the wing from stalling
on gliders that are prone to wing dropping.

Also reduces lift (AoA) and helps glider stick to the
ground, ie, no desire to keep flying as the wing can't
generate enough lift.

John

PS Some glider have ailerons that go negative (or do
not go as positive) as flaps are increased towards
landing, eg, Mosquito, ASH 25.