In a way mailing lists are like subject-related discussion groups. Every message being sent to the 'group' is automatically e-mailed to everybody whose name is on the list. So people are always up to date with the ongoing discussion.
Discussion lists are usually a good source of information on a particular subject.
How broad or specialised a subject is discussed depends on the list members. But in any case, people interested in a certain subject area will benefit from this source, whether they actively contribute to the list or just lurk around and read the information available. For instance, if you are interested in music and what's going on, an appropriate discussion list will inform you about say upcoming concerts, ticket purchasing details etc. The comfort of participating in a discussion without having to go anywhere to read or retrieve the contents of a discussion. It is automatically sent to you by e-mail.