Great, so you can quickly switch accounts, but why would you want to, especially if you’re just one user on one Mac? Here are a few possible instances for using multiple accounts, beyond the parents-with-kids scenario.

Smart advice from Macworld. I'd add one thing: Having Guest login available means you can allow someone else to use your Mac to get something done, with no danger of them accessing your files, screwing something up, or leaving their passwords and cookies in your web browser. When a guest account logs off, all the session data is deleted.