What do you mean what is it useful for? Do you not know what the Get Mouse Position UI X,Y does?
How are you suppose to differentiate a position of 500,0 in a 1280x720 from a 500,0 1600x1200 if you don't know the resolutions before hand? How do you standardize the responses to the relative positions?
I thought this would be more then obvious. I can already do it indirectly, exactly what someone said before, ask and pray, but i'd rather have a direct method, one that doesn't rely on a newbs ability to correctly select his resolution, or even make sure all supported resolutions are listed (if its a drop down list), or to type it in correctly with the correct format.