Q: Why do horses paw at the ground before they start to roll?

A: Horses paw before they start to roll to create the ideal patch for their rolling pleasure.

Pawing helps to loosen the earth or sand which a) creates a softer surface to roll on and b) allows them to expose the slightly cooler earth which then helps to cool them down.

NOTE: Pawing before rolling is a natural behaviour, but if a horse paws and then rolls in other contexts then you need to consider whether the horse is showing colic or some other kind of pain.