Useful if you want to go into the files for extensions and make changes, e.g. such as changing install.rdf max version number so extensions work in newer versions, removing locales other than the one you are in (for me, en), etc. And, useful when a new version of thunderbird comes out. You can check the version numbers that the extensions work with. Why upgrade if your extensions won't work?
Speaking of which, Extension Manager Extended says it works with 3.1, but the max version in the install.rdf is only 3.0 as of today, November 1, 2010.