I'm not a FitBit user, but I think for a group of people (myself included), being able to measure & track the meeting or exceeding (or not) of goals can be a great motivator for continued work/improvement. For example, if I step on the weight scale regularly and record my weight (in a simple app on my phone), I feel more motivated over a longer period of time to make better dietary choices as well as stick to regular exercise. Without doing that, my will-power feels less and it's easier to take the path of least resistance and ignore the consequences of my (lack of) action.

I'm not saying you need a FitBit for this, but if it enables this kind motivation in a convenient way, that could be well worth it.