Our door locks itself when you pull it closed. Obviously we can open it from the inside but you need a key if you're outdoors.
But then we live in a house, not an apartment, so maybe it's different for that type of door.
My front door is the same.
My backdoor is different. When it's unlocked, it can be opened from the inside and outside. If I'm down stairs I don't normally lock it but if I'm going to be really distracted, I'll lock it. The only reason is that occasionally people come into the garden, normally on the run from the police and they go through people's gardens before running into the woods. And next door's brats who seem to consider our garden part of theirs, despite the 6 foot fence around it.