I recently acquired a fitness tracker aka smart watch and have been tracking steps, walking distance, heart rate calories burned and other health or fitness related aspects. The most useful I have found to be calories burned in combination with recording what and how much I eat as well as how much Water I drink during the day. As opposed to the probably more common goal of trying to lose weight I discovered that I have the opposite problem. My mostly healthy diet struggles to keep up with my rapid energy consumption, which I never realized before. I am now more aware of what I eat than ever before.
When not giving it particular attention I also most definately do not drink enough water, some days even less than a liter. Logging how much I drink helps me increase my water consumption, though I am nontheless struggling to meet my dayly goal of 2 liters.
Other features like the step counter are neat features but personally havn't been of much use other than satisfying my general curiosity. I also refuse to use the social features the companion app offers as I have no desire to enter a competition of who takes the most steps in a day with my friends.
Concluding I would like to say that collecting data can be the key to solving problems or improvement of certain areas in life. Excessive use on the other hand can also be very annoying to the point of impacting social life. But then again excessive use of anything is never a good idea.