WHY do I need it ?

Whenever you need to modify the UV mapping of an object AFTER you have built its texture, you need UV2UV to fit this texture back to the newly mapped object.

Imagine you have a nice texture that suits an object for which you need to rebuild the UV coordinates.

If you reuse the same texture after UV manipulation you will probably have matching problems.
UV2UV will redraw the new texture for you with a very efficient result.

With UV2UV the choice of the mapping method (projection mode) is no more irreversible and can be changed at any stage of your texturing process !