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 with velocity makes sense for after-touch capable synths.

Answer 2

1. see wwwar's answer - it's for after-touch
2. yes, NoteOn with zero velocity is essentially NoteOff
3. this is simply because that is what a lot of MIDI keyboards send out. You can do either according to the MIDI spec. The advantage of using note on with zero velocity is that it can be sent more efficiently if you use MIDI running status