Potentiometric titration using a pH meter
In principle, you should know the pH of the buffer solution. Measuring it before an unknown sample is a straightforward way of calibrating the detector (pH detectors can get a bit funny after a while). 

If you put the detector into a pH 5.5 solution and it reads 7, you're going to have to tweak the detector (if possible) or find another. 

Measuring unknown samples with a detector that incorrectly measures a known buffer solution is a bad idea but an even worse idea would be to measure unknown samples without knowing your detector was incorrect in the first place.