Head temp even on tractor prop planes, is affected by prop size, and other factors.

Knowing the head temp helps to make sure you are not prematurely degrading the engine.

The "spit test" is not quite enough, nor are the engines designed to run hot.

I've seen all too many people tune up glow engines for best performance, back off a bit as they are supposed, only to have them seize in flight resulting in a deadstick or a crash.

While using the wrong prop was ultimately the culprit, monitoring the head temp would have pointed to the problem.