Someone posted in the /r/Apple subreddit an interesting story how his Apple Watch saved his life. It appears that the hear rate notification warned him right before things went "nuts".
I was laying in bed, enjoying some TV and homemade brisket, when my Apple Watch told me that my heart rate was weird af, and then, told me my heart rate was stupid fast (thank you heart rate alerts)
Called ER, when they arrived, they found me in serious trouble. Body went into shock, got rushed to the hospital in a stretcher, and got taken into trauma.
I felt totally fine before everything happened, and then notifications, and then BAM, everything goes nuts.
The heart rate alert warned the user about an abnormally high heart rate in a resting position before he called the ER. Upon waking up, the doctor administered an ECG, which turned out to be in line with Apple Watch's "diagnosis" - abnormally fast heart rate while all chambers of the heart were beating normally. He was then diagnosed with supraventricular tachycardia.
The original poster on Reddit says that the watch saved his life because if it weren't for the early heart rate alert, it could have ended badly and couldn't call the ER in time.