Suppose you want to configure a Linux app to send out email messages from your server or desktop. The email messages can be part of email newsletters, status updates (e.g., Cachet), monitoring alerts (e.g., Monit), disk events (e.g., RAID mdadm), and so on.
While you can set up your own outgoing mail server to deliver messages, you can alternatively rely on a freely available public SMTP server as a maintenance-free option.