Good question! Of course, the obvious answer is “because good drivers should be rewarded”. But there’s a bit more to it than that.
Bad drivers have more accidents. More accidents means more insurance claims. And more insurance claims means higher costs for insurers (that’s us).
Safer drivers, on the other hand, have fewer claims. Having fewer claims saves us money. So if you can show that you’re a safer driver (say, by using Marshmallow Miles) then it’s only fair we pass those savings on to you.