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).
Garmin has a new wearable in the works. The Garmin Cirqa has already surfaced, and its launch can be considered certain given official certifications. However, it is still unclear what added value the wristband is supposed to offer. Some informed assumptions can already be made, though. The question of its purpose is certainly worth asking.