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Web dev: What browser is visiting the page?
User agent string:
A screenshot of a browser. The URL bar reads firefox://settings, a button on the URL bar is labelled Netscape, a popup from the button reads: "You're viewing a secure Opera page", and the web page title reads "Chrome settings".
There are some use cases other than web page compatibility. One for me is in dealing with firewall and proxy policy, if the agent is a browser and comes in on specified explicit ports then force authentication, things of that nature.