The watches have suggested people go to hospital due to arrhythmia, with life-saving results more than once. The new Apple heart beat sensor supposedly allows this (so you won't find the same functionality on Fitbit etc.).
Yeah, Apple aren't known for being entirely honest about the scope of their innovations. Suffice the say, that's Apple parlance for "a handful of times the past year."
It's likely they saved lives with previous models too. I work at Fitbit (don't speak for Fitbit) and we often see stories of people who say their Fitbit saved their life because it helped them notice some heart condition or other.
Pity you require an iPhone to even turn them on.