Because it's awesome! You've never had to rewrite a query for some sort of reason other than the results returned (e.g. performance) and really wondered if the original and rewritten query were exactly the same?
It's not formal verification by any means, but it seemed to work pretty well in practice. It would take forever though. I'd start it and walk away for a couple hours when I wanted a thorough test. But my confidence in correctness and performance gain (it measured perf too) was well worth the effort.