I once went to my colleague’s housewarming event. When I reached her unit, I tried to press the doorbell switch which is located outside. However, I couldn’t hear any sound and I just knocked the main door instead.
Shortly after I knocked the door, the host appear and she explained to me that they are not going to install the doorbell because they need privacy. Well, it is really up to the person, whether he / she prefers not to install the doorbell. It is really up to the individual.
However, I really need to emphasize the importance of having doorbell to the visitor. Without the doorbell, visitor will also need to knock the main door and it might create damage to the main door. This can be avoided if you install the doorbell.
Thank you and have a nice day!