Have you ever seen the Synth1 manual? Because that's exactly how vel is described in there.
"vel Adjust how much the amplitude changes according to the velocity of the note played. Turn left to decrease the sensitivity and thus the volume change; turn right to increase it."
Not easy to know why it's in there though. My guess, remembering how old is the Synth1 design, is that it was a reaction to early MIDI keyboards which used to only have a few steps between 0 and full velocity so a slight change in how you pressed the key could cause the velocity to suddenly drop from really loud to really quiet. But it's just a guess.
The manual is at
http://www.geocities.jp/daichi1969/synm ... meeng.html It's not the easiest read but there is a surprising amount of useful information in it.
Steve