I've never once been in an actual hotel, and definitely not as a paying guest, so I base this entirely on 50+ year old movies and TV series, as well as various remarks and books/stories.
It appears as if you "check in" when you first get there, and then get a key to your room. Then, when you are going out (if you are going out during your stay), you are not supposed (or perhaps even allowed) to bring the key with you. Instead, you have to leave it in the reception and then, when you come back to the hotel, ask for it before going to your room.
Why not skip this seemingly pointless "middle-step" and just bring the key in your pocket, as it's "your" room for the moment as you are paying for it? Why require each paying guest to waste their and your own time with this "centralized storage" of the key?
And does the receptionist memorize with their photographic memory every guest and their room number? Do they actually ask for identification/passport each and every time you come back to ask for the key? If so, that sounds like an even worse waste of time and energy. And if not, what if you just say somebody else's number to gain access to their room? Or if somebody just enters the hotel and pretends as if they live there and ask for the key to room number XYZ?
And I thought the hotel snatched your passport/id/papers when you check in anyway, so you can't exactly show them to the receptionist in the scenario above anyway. And what happens if the receptionist with the photographic memory goes off his shift and somebody else stands there when you come back?
I have so many questions about this. It seems like insanity. And if it's different today, I'm still wondering about this in a historical context.