Note-off velocity means the speed with which a pressed-down key is released back up. This information can be used for at least Rhodes electric piano sounds, so that the keys make a different release sound depending on if the keys are released softly or ”hard”. The difference in sound isn’t big, but there is a difference. An experienced player can notice the difference in sound, and loud key releases can be used for certain types of rhythmic “barking” comping.
If you read the MIDI Implementation Chart of instruments, you’ll notice that many, or most keyboards don’t send note-off velocity. Even the Nord Electro keyboards, which are supposed to specialize in accurate emulation of Rhodes elecric pianos and other comparable keyboard instruments. Some sample libraries have multiple layers of sampled note-off i.e. key release sounds, so by using a keyboard which doesn’t transmit note-off velocity you’ll miss out on that dimension of expression.