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#11
The Fn key is for keyboards that have custom functions attached to their Function keys. Most laptops will have screen brightness, WiFi on/off, etc attached to their function keys. Sometimes there is a setting in BIOS that determines whether a function key, by default, will output the F1 function or the Mute function (for example). The Fn key toggles it so that the opposite function is output by the keyboard.
Most desktop keyboards will have all the custom functions as separate keys, not tied to the F1-F12 keys, therefore they will not have an Fn key. Although, looking at Logitech keyboards, most of them do actually have Fn keys. Maybe it is not as uncommon as I originally thought.