Apartment building question, was I right to ask why do you want to come in?

Don't you have one of those things on the door so you can open it a bit and see who's there before you really open the door? You're right to be cautious because after all, anyone can say they're the fire department from behind a closed door. But once I knew it really was a fireman, I don't think I'd be asking why they need to come in. Firemen don't ask to come into people's apartments just to check out how they're decorated.