Hey PC holly! Welcome to DC.
I am not sure why people DONT use fastpasses all the time, but I’m guessing it’s because people don’t realize they are free, or just don’t know in general.
Is there a limit? Yes and No. You can get as many fastpasses in a day as you want, but you can only have one at a time. Usually the fastpass will tell you when you can get another one.
How does it work? Basically, you go to the fast pass area near the ride, slide in your ticket, and out comes a fastpass (and your ticket - be sure to NOT forget your ticket - that would be a mess!)
The fastpass will have your that days date, as well as a time to come back for the ride. IT’s usually a ‘window’ of time - such as 11:35 - 12:35.
You come back at your scheduled time, go to the FastPass enterance, and you may have a small line wait - but, usually that’s just minutes.
I hope this helps.
Oh, and just so you know A fastpass is about a little bigger than a theatre ticket stub.