Club Speed is the speed the club head is traveling immediately prior to impact, it determines a golfer’s potential distance.
More club speed equals more potential distance. In fact, adding 1 mph of club speed can increase your distance by up to 3 yards with the driver.
It's useful to establish what is possible at 100% effort, but also what speed produces the best all round performance of control and distance. Sometimes swinging slightly below your max, helps you find the middle of the clubface more often, and the average ball speed is better.