I bought this wrench and another bigger one like it at Big Lots 30 years ago. It has been one of my favorite tools.
It enables me to apply a lot of torque to a nut or bolt without the need to get out a socket set or find the box-end that is the size of the nut, which I might not have anyway because my biggest regular wrench is only 15mm. It gives a good feel to be able to lock it on cleanly so that the nut is protected from rounding. It adds a layer of stability, which helps leverage. I recently clamped it on brake line flare fittings and it was able to loosen the connections very easily. I use it most often on bicycles and I also particularly like using it on oil pan drain plugs.
Stanley has a similar one now, and I plan to get it soon.
If someone has that Stanley locking crescent-type wrench or another favorite tool, especially an unusual one, please post the picture if you can and describe how the tool has helped on projects.