It seems a lot of folks are confused about the purpose behind PIDing an espresso machine. A PID, in this application, has but one function; managing the temperature of the water in the boiler at a precise temperature, so that the water used to pull an espresso shot is the same at the beginning of every shot. That's it!
Manage the boiler's temperature so each and every shot begins at the same temperature, to within <1°F. Pretty simple stuff to be creating such interest, wouldn't you say?