You need to do this if;
1) Your watch is new
2) You setup from new
3) You’ve never done this before
support.apple.com
1) Your watch is new
2) You setup from new
3) You’ve never done this before
Calibrate your Apple Watch for improved Workout and Activity accuracy – Apple Support (UK)
You can calibrate your Apple Watch to improve the accuracy of your distance, pace and calorie measurements. Calibrating your watch can also help it to learn your fitness level and stride, which improves accuracy when GPS is limited or unavailable.
