There are several reasons why it's a good idea to regularly remove surf wax.
Fresh wax job improves grip. A fresh coat of surf wax will offer a much better better than a two year old wax job complete with dirt, sand, embedded empty beer cans etc. Why not go the whole hog and even do a base coat first, you won't regret it.
Cleaning your board down will make it possible to identify dings that need fixing before they become an issue. Recently we had a post on the hardware forum from a regular who'd noticed a dent near the tail of his board. Following the removal of the wax he found a crack in an old repair that had started to let in water. This crack, left unchecked, would have continued sucking up the water and caused increasing damage to the board. Without removing the wax it would have done undetected, sucking in more and more water. Read about the find and repair here.
Perhaps the most important reason for some, a freshly waxed board looks nice.
Now, what to do with that old wax? (stay tuned!)