View source for Talk:Go back to an old version of Thunderbird

I don't understand the context and need for this sentence "If you backed up your profile before upgrading and still have copies of any new messages on the mail server then you don't need this article. Just download the older version from [ftp://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/ here] , install it in a different directory, and then restore your backup."