I’d say it’s a bit of both. It depends on what you are trying to do with it, and on how well the solution is implemented. A good sleep tracking smartwatch like the ones found on this link will be able to provide accurate reports on how much sleep you got and how it compares to your sleep patterns on other nights. And that alone can be very useful, especially if you have an uneven sleep schedule that makes it hard for you to keep track of when you have slept too much or too little.
This also helps shine a light on potential health issues. If you are sleeping 5 hours a night and feel tired all the time, that’s to be expected. But if you’ve got 30 hours of sleep over the last 3 days and still feel exhausted, you might need to see a doctor.
On the other hand, sleep tracking isn’t always accurate, especially on cheap watches. And a lot of watches will try to give you some sort of objective “sleep quality” score, which isn’t always trustworthy. So how useful or gimmicky sleep tracking is will always depend on how you use the data provided by it.