Doors are mostly responsible for most of the noise issues you experience, which is why they are given particular attention in acoustic treatment. Like a window, a door can be a gateway for unwanted sound to enter a room. A door creates air gaps, which allows the entrance of unwanted sound easily.
Why would you want to soundproof a door?
Perhaps you live with lousy flatmates, and you really cherish your quiet time or maybe your apartment share a door with another rented apartment and you would like to maintain privacy. Whatever the reason maybe, you can regain the privacy of your space.