Have you tried to stick anything metal into one of these plugs? It is fairly todler proof. I think a 7 year old has the dexterity to defeat it but certainly not a baby.
Manufacturers sell the wall sockets in both switched and unswitched versions so I would say the switch is just there for convenience of the user.
It seems that maybe in the 60s having a switch on the wall was very useful because most electrical devices would have been designed to be switched on or off from its mains power connection (like lamps, hairdryers, vacuum cleaners, electric whisk, etc). I assume the standard was to have a switch as this gave a little extra functionality. Today however many electrical devices have digital electronics that don't expect to be power cycled and so a switch is not really a feature but an inconvenience (think smart bulb, or Apple TV).
Agreed, most people I have asked also think the switch is an additional safety feature. I can see that it is helpful to isolate a device and so can make using all manner of devices safer, but it requires that you know what the switch does and when and why you would switch it so this kind of excludes babies and toddlers.
My wife is from North America and common knowledge there is that wall sockets are just incredibly dangerous at all times. The UK style plug is quite different.