Basic 3 question

Yeah i already beat the mission, but looking back on it, i guess i did learn how to change user agents, but i still don't understand the purpose of a user agent.

They are a browser's way of identifying itself to the web server. You can use PHP (and other web languages) to display different content based on user agent.

For example, if the viewer's browser is IE, you can load an ActiveX object, and if it isn't IE, you can display a message telling them that they need to use IE for the ActiveX to work.

help me plz if you have completed this mission downloaded a addon to firefox for useragents but i still cant work it out :(