I used to live in an apartment, 2nd floor, above a bank, so I felt safe already, lol, building was secure with alarms and all that.
And even if I would just go accros the street to get a bread from the machine or mail a letter, I simply just couldn't leave my apartment door open (only 4 apartments in the building) knowing there's also a main door no one who didn't live there could get thru.
And, when I'd go to bed, I'd also leave the key in the lock not that it would probably do much, if they wanna be in, they will, but it did ease my mind.
When I lived with my ex, brand new front door, but even with the key in it on the inside, you could still open it with another key from the outside
I hated that, and made me feel quite unsafe when being home alone in wintertime when it was dark early.
He was used to people just knocking and coming in straight away, but I was brought up with a lot more privacy where people wouldn't do that, so I hated it when someone would just appear 'round the back when I didn't hear them knock. So all doors always locked.