Some people like turning things into games as a mechanism to achieve certain goals.
For me I use it more for convenience. There is good evidence that logging your behaviour promotes healthy behaviour. With fitbit I can log my food intake and it will automatically log my activity and weight. In the past I would have logged weight manually in an excel spreadsheet and probably just not tracked activity as closely.
Being able to enter your meals from a shared database makes this process so much smoother, and having a rough idea of how many calories you've burnt so far lets you make smarter choices about food.
Also, one of the things that the Surge model can do is track your runs via GPS. This is awesome. At first it was the only reason why I even got it, but I think the whole feature set is really helpful toward reaching your fitness/health goals.
I will say that it was neat and then lost interest, but later once I had some "fitbit friends" the gamification went up a major notch as has the stickiness.
But within a few years either FitBit bands will turn into watches or users will replace their FitBit with a watch. So from the FIT perspective it hardly seems like a stable investment especially at the current market cap.
Seeing the data from my workouts and activity, even if not 100% accurate, helps me quantify the work I put in and the results I get. Removing some of the abstraction and seeing the numbers makes the hard work more rewarding.
I use a Xiaomi MiBand, the battery lasts for 30+ days and it is cheap. Just wear it and forget it kinda.