The elevator button allusion is perfect.
The calibrate every time school of thought is based on:
• fear of losing control of the Phantom
• a lack of knowledge of how the Phantom works
• a desire to do something and
• the concept that if something is good then more of something should be even better.
It's the same principle used by the ancient Mayans who sacrificed a virgin each day to ensure the sun would rise the following day. It's superstition driven by fear and ignorance.
Calibration of the compass is necessary because of magnetic declination which is the difference between magnetic north (based on the Magnetic North Pole) and map north (based on the True North Pole. Declination is different for different locations and maps are available to show what your local declination is.
You have to calibrate the compass in your Phantom because it was assembled in Shenzen, China where the magnetic declination is -2° and it has been calibrated and tested there. When you take your brand new Phantom out of the box at home the declination is likely to be something different. The Phantom needs to know about this so that when it can allow for the magnetic variation and know where it is flying and how to get back to its home point.
An error of 1° is not likely to make much difference but an error of 10° or more would be significant. Looking at maps of magnetic declination you can see how far you would need to move to make a significant difference. Thirty miles won't make any difference unless you fly very close to one of the magnetic poles. If you travel along one of the contour lines of the mag. dec. map you could travel quite a long way and still have the same magnetic declination. For example from Minneapolis to New Orleans or Norway to the Congo in Africa.
Here are some maps showing how magnetic declination varies across the earth. Understand your Phantom and fly better.