The purpose of a bike fit is to find the right balance between aerodynamics, power output and comfort, so if you use your bikes differently, it follows that you will have different fits for different bikes. As an extreme example, your hour time trial bike will have a very different fit than the beach cruiser that you use for short rides around town. I have a bunch of bikes and they all fit a little differently depending on what I use them for. Some of my bikes even have different stems that I swap depending on the duration and intensity of the ride.

I've personally never had a professional fit, but I think it's a good idea in 3 cases. 1) You're new to cycling and need some help or want some advice setting up your bike 2) You're racing and want to eek out every available watt of performance 3) You're having some discomfort and are unable to resolve it on your own through trial and error