So in doing research, I've noted two types of low riding front pannier racks: some have a bar that loops up and over the tire connecting the racks on each side of the fork, and some do not.

What is the purpose of this bar, and is is more desirable to have it or not? Is it just for more rack stability? Mounting lights? There doesn't seem to be any correlation to price and it being there or not, i.e. it doesn't seem to indicate quality.