Cleaning your yoga mat helps ensure good hygiene, since your hands (face and feet too) touch the mat in various poses, and then they touch a lot of other surfaces too. Your feet can even develop infections from a dirty yoga mat, including plantar warts, athlete’s foot, ringworm, or staph infection.
Another important reason to clean your mat regularly is the safety factor. You need to make sure there isn’t any sweat build-up on the surface of the mat. Sweat build up can be slippery, potentially causing you to slip and fall.
And finally, regular cleaning increases the lifespan of your yoga mat. When you take care of it, it takes care of you! Otherwise, without frequent cleaning, the mat will become irredeemable at some point, and will have to be thrown out and replaced. While you probably should replace your yoga mat at some point, you shouldn’t have to do it frequently if you care for it well.