Why do you put compost on top of soil instead of mixing it in?

You can mix it if you want but thats hard work, so let the worms and other beasties that are in the soil, drag it down into the soil for you over the winter months.

I usually rake it in a bit to stop it from being blown away if it gets windy as compost tends to be lighter than the surrounding soil

you don't add compost unless the soil needs it and you don't just spread it out..you either till it in and rake it out together or you run a power rake over it , which would rake it into the soil