Tappet clearance is the manufacturers designed dwell time required for the valve to sit firmly on its' seat dissipating the heat out of the valve head to the water jacket. Exhaust valves in particular suffer from burning if the heat is not allowed to be dissipated out of the valve head during operation. The only opportunity for an exhaust valve to do this is while it is firmly closed on the valve seat. If this dwell time is shortened or the seat pressure reduced exhaust valves will run so hot that they will be burnt or melted away.