There are times when you wan to send a message to the users logged in and using a server machine. The classic wall command is useful for that scenario. The message you send will appear at every interactive shell of every user on the machine. The best thing about this command is that no special chat or messenger client is required.
$ sudo apt install bsdutils
$ wall "hello folks, server will be shutdown at 1PM"
It would have been nice if this command could be used to address one specific user or a subset of users, but that is not possible.