In flight simulator X-Plane 8.15, the F-4 Phantom 2 can fold the tip of its wings (well, variable sweep). Is that useful? Why would a pilot fold the wing? To reduce lift for short landing?
The main purpose of this was simply to save precious space on aircraft carriers, it had no use when flying. There have been some incidents where wings have either started to fold during flight or where the pilot forgot to unfold before takeoff, but it was not designed for this. Many other naval aircraft have the option of folding their wings in a similar manner, such as the F/A-18.
The F-14 Tomcat uses a completely different mechanism to adjust the wing sweep while flying, and although it had a special "parking setting" for the sweep, it was used to adjust the wing sweep according to the speed of the aircraft.
The A-7 Corsair II and the F-8 Crusader also had wing folding, but in addition they could also adjust the attitude of the wing while flying, primarily to improve visibility for the pilot when landing as seen below.
In U.S. Navy and Marine Corps F-4 Phantoms, the wing fold mechanism was controlled and powered by hydraulics, since the pilot had to fold the wings after landing for taxiing around on the deck. The USAF versions of the Phantom, on the other hand, had this hydraulic capability removed, and there was a pin in the top of the wing, (if I recall), just inboard of the wing fold, which, if it was protruding, indicated that the wing fold was unlocked. It was, obviously, a critical pre-flight step to check that this pin was flush, not protruding.
There was at least one incident where the aircrew failed to notice that the pin was protruding, and took off anyway. Needless to say, the wing folded shortly after rotation and liftoff, with catastrophic results.