It would help in lining up UV's quicker when the UVs you are lining up are on the far right..

Example: the image on the right if the tool was reversed.. the X-Axis would point the negative direction.

dunno if I am clear about my question..

But notice the image on the left you can use the tool to line up on right the.. but when you work on the right side.. there is no "left direction" to help you line up the UVs.