The calibration of the expression pedal on our FBV series control devices is to ensure that the minimum and maximum (heel down and toe down) positions are sending the correct values to the amp or POD device that you're trying to remotely control.
If the amp doesn't properly "know" when the expression pedal is at heel down (minimum) or toe down (maximum) positions, something like a variable effect like a Wah won't sweep properly to give you the full range of expression pedal travel.
You don't have to do it regularly. It's good to calibrate it when you first connect the pedal to your amp or POD to "learn/teach" the amp to see the pedal range. Once you've done it, you're set.