Purpose of MP3 Tag?

Sorry for my ignorance, but I need to know this. Does merely changing the extension from another audio format to MP3 allow me to play it on an MP3 player? If so, then I understand why this application is so valuable. If not, for why would I want to use this?

No, mp3tag does not change the format of a file or otherwise do file conversions, etc. mp3tag is a powerful metadata tag editing program (you know, album, artist, genre, etc.). It works with much more than mp3 files (FLAC, etc.).

If you want to convert from one format (shn, flac, etc.) to mp3 format, you should try FOOBAR2000.