This is an issue of our industry default control being an analog slider so you need a switch point in the console. Back in the day, 0-10V control was also typical so you need a switch point in the rack. There is always the possibility of getting a transitory level so you can't depend on only 0 or 100%. 50% is as far away from the "normal" settings of on/off as possible. I last used 'on @1%' when I wanted something to be 'first on, last off' as it was a semi-safety issue. Sure I could have done other console tricks, but that was best in that situation.
I'll let others address the electrical engineering questions. Thank you @STEVETERRY for being far clearer than most EEs I've known!
I was actually thinking in terms of 0 - 10 V analog console. What I meant by 0% and 100% is where the operator should position the board's sliders for "off" and "on", not the threshold level as determined by the relay box's design. (Anything safely away from the extremes of the signal range should work as suitable thresholds at the rack.)
Though I think I see your point now (correct me if wrong) about having a small bit of "controlled sequencing" (first on, last off) across multiple devices when sliding the master X or Y? Or if multiple relays (for multiple devices) are slaved off the same control channel?