If it fits in a watch a lot of nondiabetics will also buy it. It can be much cheaper that way.
There's a theory that says you basically won't ever gain weight if you prevent your blood sugar from going above a certain level. So it's an objective way of knowing how much to eat and when.
Also, to warn when blood sugar is too low. Some people (myself included) often get so into work (or whatever) that we forget to eat, with adverse consequences. An alert is very helpful.
Usefull? It is if you use it. I do triathlons and knowing exactly where my blood-sugar level is at would allow me to focus better on the type of nutrition and the impact of it while working out. It would also tell me if i was a bit down before a race, so i can take some food.
Basically: this is a game-changer for amateur athletes, which would create a tremendous market for it. People i know already use the patches to measure as well, or lactate measurements, ketone measurements, etc. and that's just at the casual amateur level.
Another application that springs to mind is knowing when to eat instead of just having lunch and sugar-crashing 2 hours later in the office.
If I am connected to my body, I can also feel it by own biological sensors. But I do see the use case, to get another data input, for those cases where I am distracted and don't pay too much attention on myself.
Preventing non-hereditary diabetes could be much cheaper from a societal perspective.
I measure my heart rate one time for fun. It was insanely high at rest. I felt perfect, full of energy but it was completely obvious i needed to introduce rest days. I conviscated the heart rate monitor and everything was back to normal 3 days later.