Not much use for traditional scale electric racers, but:

Mechanical brakes with adjustable mixing off the throttle channel for a 1/8 scale electric conversion.
I’m not sure, but I think some 1/5 scale guys have separate front and rear brakes, adjust front/rear bias.
I think some may use it for adjusting nitro fuel settings.
Dig function on a rock crawler?
4 wheel steering monster truck.
Remote nitro starting.
Nitro reverse as mentioned.
Don’t some monster trucks have 2 speed trannies that you can shift with a servo?

OK, so not all are racing per se.