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.
It's common in the U.S., especially in apartments, to have rooms with no light fixtures. Instead, there will be one outlet that is wired to a switch by the door. That outlet is sometimes upside down to distinguish it from the other outlets. That gives you the option to connect a lamp to the switch to get the same result as having a light fixture. I would generally prefer that every room has a light fixture on the ceiling, but this is marginally better than having to walk across a dark room to turn on a lamp.
Here in Australia every house I've been in that has an electrical connection has had a light of some form mounted on the ceiling of each room of the main structure.
It just shows how any assumptions we might make will be proved wrong at a global scale.
As I understand it (thanks technology connections), the sockets linked to light switches are made that way in case you want to have like a floor or desk lamp and turn it on when you enter the room