You can also use it during the weight loss time to get a feeling for it's accuracy for your usage, to be prepared for going to maintenance.

For instance, if you took a look at each week of Fitbit report, or a month of data, and Fitbit average daily TDEE was 2500, and you logged accurately eating 2000 daily average, but you lost more than 1 lb weekly, then the TDEE estimate was lower than reality, because you actually created more deficit than the math says.
Or you actually ate less than logged.

I have been using the FB for about 18 months and hit maintenance about 8 months ago. I find that its pretty accurate in that if I am really active i get more cals to eat and if I am less active, I eat less. Its easier for me to maintain this way than having a set cal goal every day.

I have been still losing a little bit bit its very slow and could be a combination of not eating enough to maintain or that I am working on body fat reduction.

No matter what, having the FB is the best tool that I have every used to maintain my weight and keep it off.