There is a German firm called Saltus who are a wrench manufacturer.
They only produce a small range of tools, but one of their specialities is these wrenches!
As I've only ever heard these called 'Saltus Wrenches' in America, I would assume that the first to arrive there were actually manufactured by Saltus, hence 'Saltus Wrench'!
'Flex head socket wrench' makes much more sense if it's made by anyone else!
As to uses, well yes, to a certain extent these have been superseded by interchangeable socket wrenches. The Saltus manufactured tools even have a hole near the open end for a Tommy bar to turn the socket in the vertical position, which I doubt few use now!
However, like so many tools, you don't realise quite how 'usable' these things are till you've tried them! They really are 'nice' to use, and make a great complement to a set of combination wrenches.
There are also some jobs that you can tackle with these thst would otherwise need a model specific tool. Some door removal / refitting jobs come to mind! I've taken doors off with these when no other tool will fit!