Mostly a useless device that tracks all sorts of body responses, often inaccurately, that has the potential to create hypochondriacs when they see natural variations in blood pressure, heart rate, sleep quality etc. that are perfectly normal.

There is an argument for using a simple heart rate monitor with a chest band, which is more accurate, and may be worthwhile for those who want to ensure that their heart is in the appropriate training rate or for those who have a condition that might be worsened if they overstress themselves.  Even then, a manual pulse reading at the wrist, although less convenient, would suffice.