I got the idea early on in using calibre that I should do a mobi-to-mobi conversion after I add a book that started out as a prc and has since been stripped of DRM. I always do it, and things work great, but I'm not sure why I do it or where I got the idea that I should.
Can anybody shed any light on it? Is it a useless step? Once I do that, I usually make an epub version, too, so I'll be able to read the book on both my sony and my kindle.