In all arenas or pasture maintenance, you need to control the air pockets for winter to summer versatility. By adding the roller (water filled to achieve proper weight desired) to your harrow, you can tighten up the surface (or compress the air out) which will allow more water to run off. The roller can be pulled by itself or in combination as a harrow-roller. By controlling water runoff, you will have less percolation - so it will dry faster, hence you can use the arena sooner (less down time).