In order to let Tinderbox import the RTF via a watched folder (instead of the email source).
@MacDav, to me it seems you’re mixing up path and URL.
The path just shows where the file (the imported mail or the resulting RTF) are located in the macOS file system. This would never let you open the original mail in your mail app.
The URL could be used to take you back to the original mail in your mail app. (see Correct url of eml records)