- you run it on a server owned by you - no nasty questions of trust and third party intermediaries using transit servers in Ktoznaetistan
- you tie it into local authentication making it easy for your users to exchange files
- it presumes that if you are a home user and are sending the file to an external user the external user is known to you
- it does some standard address verification if the external user wishes to send a file to an internal user - again there is a presumption that the recipient will know/recognise the email address of the originator
I also guess it would be possible to add shibboleth authentication as an option for originators for added verification for cross institutional collaboration...