They have some headers and a body, and they are transported over the network from a sender to a receiver. This is useful in several contexts. Users who lack access to a news server can read a newsgroup as email. Users who aren't subscribed to a mailing list can read it as news. Articles posted to moderated newsgroups are routed to and from the moderator as email.
Policy is implemented separately by each application.
The goals are to handle the details common to all gateway applications, and reduce the amount of code that must be written in each application. As always, programmers should consider using existing modules in order to reduce the amount of code that they have to write. Programs that handle mail and news can have subtle bugs. They may make assumptions that are valid for most users, systems, and networks, and then fail in rare instances where those assumptions don't hold. In the worst case, they can create infinite mail loops and flood servers.