req.dataRetHandler is a function pointer, so it sets req.dataRetHandler to the address of the function SetFOO(). So you can call req.dataRetHandler as if it were a function (using function pointer syntax, of course) and that way you don't have to have a big if/else if ladder in the code deciding which of multiple functions to call.

But the code in its current form doesn't actually call req.dataRetHandler, so in effect the answer to your question is "nothing significant".