Based on what I've read, it's simply to firm up the seed bed which supposedly yields better germination. By broadcasting directly on loose soil, a percentage of seed will likely get buried too deep and fail to grow. I think the rule of thumb for seed planting depth is approximately 2 times the diameter of the seed. When you look at it that way, it's easy to see that a small seed like clover cannot withstand a 1 inch planting depth. Firming the seed bed first will ensure that less of your seed gets planted too deep, thus increasing your germination rate, in theory.