comparing ebook formats

The Calibre manual seems to indicate you can only compare the same file types of an ebook. Why would you have the same format of an ebook, twice, in your library?

I thought the conversion process was to convert one file type to another, like .mobi to .azw3?

Is there a way to compare one fiile type of a book to another file type of the book, like .mobi and .azw3?