Messages can be anonymous, or users can generate an account.
Other users (or the same user) can post follow-on messages to each message.
The site can show all messages to "the wall", but would also provide contextual links to other message recipients e.g. "the Berlin wall" or "the wall in my basement".
Why would people use the site? To express themselves. To hide messages for people. To rant at a nameless bureaucrat or a bad driver. To communicate with a group of people who have not yet organised as a group.