As far as i can tell, a floodgate works mostly like a door - a door can be made floating (although with more work, because it needs an adjacent wall to build), and it prevents fluid from falling, just like a floodgate.
The notable differences i know of are
- delay: doors react to signals without delay, floodgates after 100 steps. That's actually useful e.g. if you want simultaneous action with a bridge.
- blockages: doors that receive a signal to close while held open (by a dwarf or errant sock or butterfly remains) get stuck in the open position. Floodgates will also not close if there's an obstacle present, but will "remember" the close signal they received and automatically instantly shut once the obstruction is removed. Cf
here, "Single-use simple blast door" .
- interface: in the "link a building" dialogue of levers and pressure plates, doors and floodgates are listed separately
If you have only few places where you want a door-like mechanically operated passage and reaction time isn't too important, floodgates can simplify the linking-up because you'll likely have much fewer installed than you'll have doors, keeping the selection conveniently small.
I think floodgates are a bit of a legacy item - they had special functionality to enable irrigation in the 2D version and were just carried over into 3D without getting a new specific role.