remember that when it comes to the ECG it isn't something that monitors you, the optical HR is still the part doing that. its the optical HR that will detect high, low, and afib. then if you have an afib warning you can use the ECG to measure it precisely to show to a physician. the sad part is that all the older apple watches could do the low and afib warnings too (after all that was what the stanford heart test study was doing, alerting you for afib) but they decided to only put it on the new watch for what you have to assume is marketing reasons. the ECG *will* help some people but it is very, very, very niche for most and will never ever be used except for the first day you own the watch (or i suppose when watchOS 12.1 comes out in fall) to try it.
I realize the Series 1 and newer can detect afib, and all Apple watches can detect tachycardia and abnormally low rate should be possible as well). I believe the ECG will be used by many more than just once, but of course some of us fall into a demographic that will use it more than others... because of risk factors like family history and also because of current or previous conditions. The main concern here is accuracy, but I’m sure there will be (and have been) tests. I’ve worn a holter monitor in the past to determine whether palpitations I could feel were something to be concerned about. The diagnosis was that my heart was functioning normally, so it was probably stress and anxiety. I still have them from time to time, and I would use the ECG feature during those episodes to provide additional data to my doctor.