[dalyduo]

I'll second what Sy said and at the risk of restating his content add:

You'll go faster because you'll relax into the seat, feel exactly what the car is doing and make consistent steering adjustments without a significant portion of your upper-body, arm and hand strength going into holding your body in place.

You don't realize how much energy goes into holding yourself in place until you don't have to. Once you’ve driven with an insert, you'll never want to drive without one.