This may not be a Europe thing, but I've seen it way more in various western and eastern European countries than elsewhere, that it feels like it might be: I see a lot of exterior (house/building or flat) doors on residences that require a key to lock/unlock on both sides of the door.
In the US, pretty much every residential exterior door will have a key lock on the outside, but on the inside, some sort of key-less, hand-actuated lock/unlock knob that is physically a part of the lock and can't be removed. (I wouldn't be surprised if key-on-both-sides locks violate building codes for new construction in many US jurisdictions these days.)
Whenever I'm in Europe (as I am right now), and encounter this, the need to use a key to lock — or especially unlock — the door from the inside freaks me out. What if there's a fire in the middle of the night, and someone has taken the key from the lock and has forgotten to put it back? Then there are inconveniences: earlier today my wife and I left the house, leaving her parents inside (they were going to leave later to do their own thing). We only have one set of keys (it's a vacation rental), so we had to leave the keys with them, and also couldn't re-lock the door for them on our way out. (And what if we'd forgotten and had taken the keys with us? They would have been locked in with no way to get out!)
In the US, we could either leave the keys with them, or take them with us: they'd be able to lock and unlock the door from the inside without them, including being able to leave it locked (again, without the keys) when leaving the house. (This latter point isn't universal; there are some doors in the US that only have a deadbolt-style lock, so you do indeed need the keys to lock it from the outside after leaving.)
This phenomenon of requiring keys on both the inside and outside just seems so strange to me. It's both inflexible and IMO at least somewhat dangerous in an emergency. The locks themselves seem like they'd be more expensive to manufacture. Can someone explain why this lock style seems so popular in Europe? Is there some benefit that I'm missing? Is there some important historical context I don't have? Or maybe they're not actually that popular, but I've just been "lucky" during my visits here…