I suppose it’s a use you never realised you needed until you have it. Here are some examples off the top of my head:
Sandwich press stays in same place, doesn’t have an on/off button. Just flick switch and no need to unplug when not in use.
Power strip for the TV is wedged behind a credenza. Need to power cycle everything plugged into it but would need to move the whole credenza to unplug it. Flick the switch with a stick or arm and it’s done.
Using power tools and want to be sure they definitely won’t turn on while adjusting them: flick the switch and no need to unplug.
Because they solve a problem that ~nobody has for ~all their sockets.
Think about it: How many of your sockets do you actually use the switch on? 10%? 5%? Less?
It’s smarter to put switches only where they are needed, after all every component in a circuit decreases efficiency and maintainability.
I suppose it’s a use you never realised you needed until you have it. Here are some examples off the top of my head:
https://www.screwfix.com/p/lap-13a-2-gang-dp-switched-plug-socket-white-5-pack/49620
These things last decades. Even the cheapest ones.
A switch should never reduce efficiency any more than the distance in extra wire (a couple of centimetres at most).
If it does I don’t know how you’ve found one cheaper than an already incredibly cheap product.