When your golf club grooves are clean, the surface area of your golf club face can effectively interact with the dimples on the golf ball, by producing more friction. With skilled technique, you should strike downward on every golf shot. Clean grooves provide the potential for more spin on the ball. The velocity of your club speed and the accuracy of your centered hit produces the proper launch of the ball. The spin on the ball (produced more efficiently with clean grooves) will provide lift, optimize hang time and insure an increase in land angle. High land angle helps the ball drop and stop on the green.
All said, clean grooves help you get the most out of your good golf shots. Keep a wet towel on hand and wipe your grooves clean after every shot. Just like the caddies on the Tour!