I do not own one but I recently researched about it because the other day while I was walking around our apartment, I noticed that more people just keep on looking at their wrist everytime they get past a block. I knew they were monitoring their steps, but I didn't know that they were so anxious about this.
It then led me to question, since when did a 10,000-step routine became the fitness goal? Where does this figure come from? An article written by Michael Mosley (there were a lot of published articles about this but I choose this because it clearly provided the information that I am looking for) revealed that the 10,000-step routine was a result of a Japanese campaign to encourage their people to be more physically active. And sure, it became an instant hit, especially to the health-conscious. However, results of an experiment aimed to challenge the effectiveness of this routine showed that doing three 10-minute brisk walks proved to have greater health benefits, as people are more likely to increase their heart rate when doing this activity. Bottom line is that people owning these trackers for a long time tend to be more obsessed on the numbers rather than the actual goal of the exercise.
When it comes to sleep monitoring, I think the concern of some people would be the accuracy because although a tracker can tell whether a person is awake or asleep, they are still prone to errors.
With all these drawbacks, I still think it is worth the investment. Fitness trackers could definitely help people who are struggling to lose weight. Moreover, I could say that the most important benefit of having one is that it motivates a person to be more physically active while at the same time getting aware of their overall health.