The only time I've seen them really used for anything was when I was looking for a TT a few years ago (before I bought my first rig.) The seller of one TT I looked at had these type of valves rigged so he could open/close the gate valves from his pickup. That way, he could find a back road, drive 10-20mph, let the black tank empty, then get back on the highway.
He did get his just desserts -- he cracked the black tank open in front of a Texas DPS officer and got a $3500 fine (and I think jail time, though the TT seller didn't fess up to that.)