I guess the problem is there is only this many places where you can comfortably wear something to measure your HR. I guess the most case for this is
1. not to be bothered by display and notifications
2. to be able to wear normal watch. At least it will not look weird if you have this on the other hand (not having two watches but a bracelet and watch).
Another factor is that screen-less wearables like this will be more comfortable for sleep tracking than wearing a watch