Its just a rule of thumb not to work a muscle if it is still sore. Like others have said, you can do light resistance or full body cardio/calisthetics such as swimming to process the lactic acid. Ive never heard of of lactic acid being a catalyst of muscle growth.
Its debated whether delayed onset muscle soreness is secondary to lactic acid production or microtears in muscle sarcomeres. I say after a hard resistance workout, a muscle needs AT LEAST 48 hours rest. The bigger and stronger you are, the more weight you lift, the more rest you need.
I also agree with the guy who said increase calories to expedite recovery.