Approximately 2/3 of the body is water, which needs to be filtered through the urinary system to get rid of waste products from metabolism. Feeding too much protein can overload the kidneys, which in turn can affect the horse in many ways eg Bucking, napping, rearing, grumpy or biting when girthed up, etc.
The 500g tub lasts 4-6 weeks. It helps to tone and flush the kidneys and helps expel any build up of lactic acid. Roger recommends dong a month's course every 6 months as a maintenance.