Very good for trolling other players in multiplayer. When entering the pressure plate opens not only the fence gate, but also the floor below.
On the picture you can see a small house which is built from the materials you can find in NPC villages. Only it is not just any house! It is a player trap. The pressure plate in front of it only opens the fence gate. At least that's what everyone else who doesn't know this trap thinks. The pressure plate triggers a redstone mechanism, that retracts a piston.