Regular external calibration helped me build confidence. I was suspicions if the watch worked, and how well it worked. Calibration is every month - it expires. I take 3 measurements in parallel watch-cuff, and tell the watch App what the cuff measured. I didn't mind it initially. I now find it annoying tbh. I see why Apple would like to get rid of it. Post calibration, I measure 3 times in parallel both watch and cuff to check if they match. If the difference is within 5% - then fine. In 3 years it's happened twice where they differed more and I had to recalibrate. From recollection - both times with a reason, a TV was turned on in living room.

I have the impression that BP measurements differ though the day (lower in the morning, higher in the evening), depending on activity or at rest of course, I think there maybe some seasonality too (winter v.s. summer). Measuring BP when going to the doctor once a year strikes me as a wholly inadequate. If the BP is absurdly high out of normal range - then yeah, we learn something is wrong with the patient. But in any other case - don't see what can be deduced from 1 random measurement.

To check if the calibration is important, I had my wife use the watch, that is calibrated on me. She got absurd readings. So yes - the calibration is important and is tied to the person that did the calibration. (edited to add this)