It's for exporting data about events, appointments etc. from your calendar into a separate file, which you could then import into another calendar app. It has nothing to do with saving old emails. If you want to create a backup of your emails that you could reload into Thunderbird then the easiest way is simply to make a copy of your Thunderbird profile: that includes all your emails, settings etc. If you want to save individual emails as separate .eml files you can do it with the 'save as' menu option. If you want to export folders you could look at the ImportExportTools add-on.