Partially for kids, partially to avoid someone getting into the car. The biggest reason is that we just grew up that way (not in the US). That said, I don't know why it's a popular feature in the US. Fender mounted turn signals are a heck of a lot more useful and those haven't found their way into the mainstream.

Many non-taxi MBs I've been in outside North America have such a feature. In fact an early 80s 280E I rode in Hong Kong had in place of door lock buttons a clear capsule with a red indicator. When the car hit a certain speed, the red indicator would retract within the clear capsule and the outside door handles were disabled. I don't know if the driver could override this feature.

You're not locked in the car because the inside door handles override the lock. Only the front doors in some cars and seemingly not any door in Japanese cars. Some Euro MBs have door lock buttons that retract completely into the door lining such that opening a door unlocks only that door. Only the driver's door handle controls central locking.

It can be disabled in the Caravan.

Sixto