I believe the occassional adjustment is to compensate for wear of the seat and the face of the valve. On the old mercededs six cylinder gas engines sometimes the seats were really back in. Part of this was from cutting the seat enough to clear any damage from the seat.

Result of burning valves from no periodic adjustment and the fact that the adjustments did not hold as well back then. Mercedes really stiffened up the adjusters about 1970 on their gas engines to lessen the problem. The tendency to reduce the clearances with wear is just a result of valve train design. Even way before that some mercedes mechanics could find no good reason that they had not installed hydralic lifters.