Some people really like them. I agree that most people who use them are lazy, and don't workout but for the small percentage that do take bodybuilding seriously and use one it is helpful. Some people are very numbers orientated. I believe the FitBit can still be off though, around 10% up or down. So I wouldn't put 100% trust in it if you do get it.
Idk, personally I don't care how much I'm burning, if I'm not gaining/losing I just adjust my calories accordingly.