Why do I need to calibrate my pH meter?

Actually you are not calibrating your pH meter. What you are doing is calibrating the pH meter with the particular pH electrode attached to it to a known chemical standard of a certified NIST value called a pH buffer solution.

You calibrate the pH electrode and meter first to buffer 7.01, (or 6.86 depending upon the buffer set you selected when setting up your meter). Buffer 7.01 is the STANDardization buffer to set the pivot point of the X-Y Cartesian Graph used to set the calibration SLOPE of the pH electrode and meter. The SLOPE is the second or third point of the calibration and sets the SLOPE of the line ( \ ). The SLOPE varies with temperature and it is necessary to calibrate the electrode and meter to a NIST known chemical standard to have a known pH reference point.