Totally, 100% plus, agree with si_the_geek & edgemeal. Renaming a file's extensions does not convert anything.

As for a reason to rename extensions? I can think of one that I have used in the past. Let's say I have a text parser/reader that only supports specific file extensions. If I have ascii files with extensions of .xyz that must be changed to .txt for that app to work right, then a project like yours could be useful. In the past, I'd simply use a command prompt and rename the files. I found that easier than writing a script or project.