- there is no one around you to call an ambulance
- you can’t call an ambulance yourself, because you are knocked out or can’t move
Increasingly large odds after a certain age, and not small if you do certain sports (e.g. trekking, climbing, etc) regardless of age.
In fact, older people falling accidents is so common, a proverb in a European country says: "The elderly die either from falling or while shitting" -- the original is funnier than the translation, because the two causes rhyme).
The accident doesn't have to be terrible either. Tons of run of the mill accidents leave people unconscious...
But you don't read about such cases as such.
E.g. an elder person who fell either they eventually get up in time and call for help (so you don't read any story), or they don't, and the story is "person found dead in their apartment" etc, and might or might not mention the fall...
Is it as necessary as a spare tire in your car or a fire extinguisher in your home? No. But (since the device it does other stuff too) not bad to have regardless.
Note: For certain categories, this is so needed, that there are expensive special purpose devices that are popular for certain cases (e.g. elderly with dementia, parkison, bad backs, and so on), e.g.: https://www.medicalalertadvice.com/fall-detection/
But this is a multi-purpose device, and a cool one at that, plus it has a phone attached and can call the 911 itself, and gives this ability as just another feature to everyone!