What is Compass calibration and Motion & distance in Location services

I own an iPhone 6s and I want to know what compass calibration and motion and distance is and what is it actually required for???

I enabled compass calibration and it seems to be using my location most of the time for some reason especially when I wake my iPhone I just have find my iPhone enabled and location based alerts etc all the rest i have disabled such as home kit and wifi networking etc But i want to know what its actually for and do i need it enabled??? Has it got to do with the compass app or maps I often use these apps but see no difference in these apps when it is enabled or disabled.

Also it seems motion and distance doesn't even do anything It seems to calculate my distance, steps and flights climbed in health app fine when disabled but i don't know what its for please can someone explain both of these??