This allows you to run an email "server" that will accept any and all emails and allow you to query them afterwards - similar to the service Mailinator.com provides.
Using this, you can test whether your applications can send emails correctly, and you can retrieve the data out of them (for example if you need to verify that account activation links exist in the outgoing email, and that they work correctly).
The real Mailinator service is better if you are only sending a one-off email...