Why is there a velocity parameter for Note Off Events?

Question

1. In MIDI note off event velocity can be specified as well, why?
2. Always when a Note On event's velocity is zero, it basically represents a Note Off event? and vice versa, always when there is a NoteOff event it represents Note Off regardless of its velocity?
3. I noticed in some MIDI files, instead of using Note Off events, there is another Note On with velocity set to zero for the corresponding Note On, why not just use simple Note Off events?

Answer 1

Note off velocity has ABSOLUTELY NOTHING to do with aftertouch!

Just like you can use (Attack/Note on) Velocity MIDI value to affect any parameter in your synth, you can use Release/Note off velocity value to affect any parameter in your synth.

Typical use would be for instance that depending how fast you let go of a key, it changes (modulates) the speed of the amp envelope release: let go of a key fast and the sound of the corresponding note fades out quickly; let go of the key slower and the corresponding note takes accordingly more time to fade out.

Answer 2

In MIDI note off event velocity can be specified as well, why?

I'm not sure of the intention behind its origins but the classic use-case for note off velocity is a harpsichord — the hammer falls differently depending on the speed of the release and changes the sound. Some sample libraries (usually 'complete'ish ones for specific instruments like piano or violin) include separate "key off" samples. Release velocity can be used to vary the level and length of those samples.

For general synthesis, key off velocity is often patched to an envelope's release time. Release quickly and get staccato notes. Release more slowly and the sound will ring out a bit. Of course, you could map it to any modulation destination your synthesizer supports. Increase the ring modulation on the tails? Why not.