The test point on a regulator allows you to measure the delivery pressure.
The modern 30 millibar regulator produces a pressure of 0.435 psig and no DIY person is going to have the right (manometer) tool at home - and it needs a properly trained person anyway...
As has been said, a special pump with volumiser is needed to test the regulator and pipework.
In a similar vein, the yellow valve next to the test point is NOT to be used as a normal use gas shut off, it is only put there for engineers to use when testing.