There's no special usecase, just the wish to see how templates/macros are expanded (though this step is probably iminent to the compiler and as such 'invisible'). However I might be wrong and there's a "solution"/trick.
I should believe that such a switch could greatly help in understanding hygienic macros and templates...
If you are writing the macro yourself, you can call toStrLit on any NimNode to render the equivalent Nim code. I usually have echo toStrLit(result) at the end of my own macros while developing.
If instead the macro/template was written by someone else, you can write a wrapper macro. Inside this macro, you can call getAst on the original macro/template to obtain the abstract syntax tree that would result by calling that macro/template, and then again make it into Nim code with toStrLit.
Both toStrList and getAst live inside the macros package