Fuel pressure question

I have a rather stupid question. What exactly does a fuel pressure gauge do? I know obviously it reads the pressure for the fuel, but how do you know what a good psi is and if it's good or bad? My motor is a mildly built 318. I was thinking about ordering one from summit. I have heard they are nice to have. But what size fitting would I need for a 5/16 fuel hose? They seem to be all 3/8. Any info is appreciated, thanks.