Without it, the car can spinout, losing you lots of time as you have to get the car back on the road. AS is there to help out in the event that you lose the back end of your car. What it does is it calms the effect of your steering input by limiting it to what you need to keep the car in control so you don't steer into a slide, making it worse. By limiting the steering, the car simply slows rather than spin out.

Because it lets you steer to the limits whilst it's still on, use it to learn how much you can throw the car into each corner before AS has to engage.