Say you find yourself using the Web on a strange computer—maybe one owned by a relative or friend, or in a library or school. When using a computer or other device that you don’t control, you can enable a private-browsing mode to prevent leaving traces of your activities. That’s good for accidental security, as it keeps the next user of the device from visiting a site with your credentials, set in a cookie or via a login, when they didn’t intend to. This an even better idea for shared computers where the subsequent user is someone you don’t know.
Likewise, you can set someone else up with private browsing on your Mac, so that they don’t have access to your settings (guest mode in Chrome) or can’t change your settings (all modes), although browsers don’t let you “lock” a browser into that mode. You get better protection by setting up a different user account on a desktop Mac or using the guest account option described next.