In web development, it can be used to quickly test displaying pages as they appear to first-time visitors.
These modes are designed primarily to prevent data and history associated with a particular browsing session from persisting on the device, or being discovered by another user of the same device.
Browsing history is not saved, and local data associated with the session, such as cookies, are cleared when the session is closed. When operating in such a mode, the browser creates a temporary session that is isolated from the browser's main session and user data. Private browsing is a privacy feature in some web browsers.
The start page for private browsing mode in Firefox.
