Most really modern bike computers use GPS tracking to calculate distance, there is no calibration.
If you add a wheel sensor, some computers use the GPS to automatically set the wheel circumference to match the GPS for distance.
Then in areas of low/no GPS signal, the computer uses the wheel sensor for speed and distance.
As a bonus, the wheel sensor can also allow the computer to register when you start and stop more quickly than solely using GPS data.
Hope that helps
Barry