Generally there are several sets of numbers along the gauge.
One set of numbers indicates the pressure.
The other sets of numbers show temperature scales.
The temperatures are the saturation temperature of whichever refrigerant you
are checking the pressure on.
If you connected your manifold to a unit that uses R22.
The needle points to a Pressure of 60 psi.
If you follow the needle to the R22 temperature scale, it will point to 34 degrees.
The saturation temperature of R22 at 60 psi is 34 degrees.
It just makes it easier than pulling out a pressure temperature chart