It's mainly useful on gliders that sit at a high angle of attack on the
ground. At low speeds, the wing is stalled, reducing aileron
effectiveness as they are in the detached airflow; selecting negative
flaps decreases the effective angle of attack of the wing and can
provide some aileron effectiveness at low speeds by unstalling the wing
earlier.

Same thing while slowing down - negative flaps will delay the wing
stalling and losing aileron effectiveness.

It also helps to get the tail up as soon as possible, although by the
time you can raise the tail the ailerons will probably be
working...what some gliders really need is an extended (and
retractable) tailwheel - start the roll with the tail level, lower the
tail as speed increases.

Just joking (kinda).

Kirk
66