For me I do workouts to train for back country hiking. So many of the workouts are timed with many rounds where you are tracking a goal. I have a Garmin that I have programmed several of the workout types into so I can see my overall time, current round time, current round count, and total count. I also use it for timed weighted hikes were I can track my elevation change and heart rates.
I couldn't think of any so I asked why people use exercise trackers. If the goal is exercise then I've managed to do without them so if you know what their purpose is then you can just answer. But if you don't know then I don't get why you're even commenting.
Others might need to use a fitness tracker for external motivation even if you don't. Extrapolating from your experience of one is rarely useful (as I tried to demonstrate with my comment).
On top of that, other reasons are obvious from any review on them (health monitoring, sleep tracking, notifications integration, goal tracking...).
