Great for turning off a device or several devices without having to unplug (e.g. if the sockets are behind a bookcase, this is much more convenient). Not a super common need but when it saves moving furniture it helps. Given that UK switches are tougher to plug in and unplug than most (due to safety features), I prefer using a switch. Also, the switches are cheap and give more options, so may as well!
I rely on one for a light where the switch broke and wasn't easily replaceable, so being able to fall back on the mains switch meant I can keep using the light.
The Libredirect Firefox extension helps