It allows me to send you a message when you are not online. You can then read the message at your leisure. You can read my message, but it is not publicly available to others. Mailing lists sprang up for the convenience of email users. This allowed the private medium of email to become a public medium for general discussion.
Every email sent to the list is sent to everyone on the list.
Mental Issues are explored on lots of these lists. Email is also the main modality used by online counselors and therapists . The ability to choose words carefully makes email a good medium for these interactions. The asynchronous nature of these interactions also adds to their convenience. There is no need for the counselor and therapist to be online at the same time.