Accurate pH measurements cannot be accomplished with a pH meter unless the meter has been calibrated against standardized buffer. Without a proper calibration the meter has no way to determine the pH value of the solution you are testing.
Baiscs of a pH meter
The pH meter has a membrane that allows H+ ions to pass though, which allows a current to flow, creating a voltage. The voltage is measured by the meter and you tell it which standard buffer it is in. The pH meter then compares the voltage of your unknown solutions to that of the buffers to determine the pH of your solution.
Standardized buffers are usually colored solutions that are guaranteed to be at a particular pH. They can usually be obtained from the manufacturer of the pH meter. These buffers are critical to the correct operation of a pH meter.
To maintain accuracy
To make a calibration curve at least three standards are needed. Without the standardized pH buffer to calibrate the meter the results will be inaccurate and useless.
To avoid drift
Most pH meters, and electrodes in general, are known to drift from their calibrated settings. It is important to calibrate your pH meter regularly to ensure that you continue to get accurate results. Drift cannot be avoided. Calibrate regularly to maintain accuracy.
To account for differences
Using standardized buffers when calibrating also helps to prevent differences among samples. Proper standards can avoid such problems as ionic strength difference and other membrane-related problems.