Having the car on a sled means no wear on the tyres. Furthermore the sled is specifically designed for the rails and acts as a guide for the car, which is more

Having the car on a sled means no wear on the tyres. Furthermore the sled is specifically designed for the rails and acts as a guide for the car, which is more reliable than having the autopilot cruise through the tunnel with the danger of the user being able to disable the AP and blick the tunnel. The sled therefore makes the Tesla some sort of a bumper car without making any damage.