well, from my experience, here is a difference: Your normal wrench, whether open-ended or not has to have some clearance in order to fit the bolt or nut. Usually, it's quite a bit actually, especially with cheaper ones IME. So when you tighten with it, you end up putting all the stress on the very corners of the hexagon, which is of course why they can end up rounded off. That's especially true of items made of softer material like brass. With the Knipex Pliers wrench, you tighten the jaws so they stay in full contact with the full flats of the bolt/nut, not just the corners, so there is much less chance of rounding them off. And there usually is no mark at all to be seen. Amazingly, if you position it the correct way around, you don't even have to squeeze the handles. And you can even "ratchet" with the Knipex too. Another very nice feature is that you can see that compared to regular "pliers", the Pliers Wrench has much more leverage built-in, look at the little actuator knob at the end of the RHS handle in the pic above, and compare its distance to the pivot to the length of the handle.
Conversely, if you have to remove a nut/bolt that has already been rounded off, and the regular wrenches can't do it because the corners are gone, the Knipex will still grip the nut tightly on the remaining faces, and is likely to get it off.
Then, of course, there is the nice benefit of always having the correct size tool for any bolt. Strangely, I usually end up having to try 3 or 4 wrenches each time before I find the correct size one, but that's just clumsy me, of course. Sure is nice to only carry one tool instead of a whole bunch of regular wrenches. Plus I bet most folks only have wrenches that go to 1" or so. Even my small Knipex (180) can do 1 3/8".
And, no, sadly, Knipex has not yet reached out to sponsor me... But, eh, I'm willing if you're listening, Knipex!!