Some of them are promoted as such. I wouldn't say mine motivates me so much as guilts me. The Fitbit defaults to a target of 10,000 steps per day, and although I knew I was a bit sedentary, I didn't realise I was often only doing 2000, which is only the equivalent of a short walk to the shops and back!
Ironically, when I'm on "holiday" I find myself easily exceeding 15,000 steps just from ambling around and about.
As a result, I've had a few days now when it's prompted me to pick up the camera and go for a nice walk, with the bonus that aside from getting some fresh air (and getting my steps up), I've been getting more use out of my camera!