A gauge will establish 'normal' operating conditions and gage variations will denote 'problems' as well as WHEN to change the filters. A gage showing errant levels will promote one to follow a logical analysis of problems. Without a gage you are forced to blindly 'guess' ... and will never know if your guess was the correct one.

Ditto with an InfraRed thrmometer that is used to profile the 'heat values' of various parts of the engine and its cooling system during 'normal' operation .... the 'numbers' will tell you faster of whats going on than 'cogitating' and being 'lucky'.