How are faders on midi keyboards used?

They don't move or anything do they? I'd hope they'd at least move, but I doubt it.
If they don't move, whats the point of of a fader thats static and stays in place once you adjust it?
 
I'm only curious as to how the different stuff on midi keyboards usually work. 

They control whatever you assign them to. If you're controlling a synth that recognizes a lot of different CC values, then the sky's pretty much the limit. Volume, filter cutoff & resonance, portamento time, fx sends, delay time, ADSR control for filters/amplitude, you name it....the list is long. Those are just some of the basic parameters you may want to have real time access to while you're playing.

I'm guessing that you mean that, unlike motorized faders, they aren't wired to mirror automation and changes on-screen.
 
They can still be useful, though. Creating automation with the mouse, either by drawing envelopes directly or manipulating an onscreen fader isn't always the most inspiring or efficient way to do things - sometime you just want to hit play and be able to get into the song and write down some automation on a more intuitive level. 
 
A bit like drawing mod wheel automation is much more intuitive using an actual mod wheel while the song plays back than with the mouse.
 
Same with adjusting synth or plug-ins.
 
At one point, I realized that I was fighting a reflex because  if I had been dealing with hardware, I'd have turned my head a bit to bring my right ear closer to facing the sound source directly, while keeping my fingers on the knob and making minute adjustments. It's something that I did intuitively with hardware - my ears wanted to take command and my eyes had to be put on stand-by.
 
When you're using a mouse, you don't really have a notion of the actual movement and of just how much you're moving things. If you close your eyes, you likely to end up with the pointer somewhere completely different moving another parameter or who knows what, because a mouse isn't anchored to anything specific onscreen. A fader or a knob can be assigned to a parameter, so no matter what happens onscreen, you are effectively controlling exactly that parameter and nothing else.
 
You get used to working with a mouse but keyboards faders can allow you to shift the focus back to what you hear instead of what you see.