At the moment in Proton Mail I have an adress book where I can add addresses of my contacts. That's it.

Beside other features like integrating with android/ios system address book or showing contact events (birthday, anniversary) in the calendar, there is one other thing that keeps us from ultimately shutting our selfhosted nextcloud down:

Multiple Adressbooks that can be shared.

Let me explain.
My wife and I both have a private address book with our own contacts. As freelancer I have a additional address book containing only contacts related to my work.
And, important for my wifes agreement to finally switch to Proton:
We have a Family Address book containing all contacts with Phone Numbers, home addresses and birthdates that are important for us both. Our friends, Families, ... much more comfortable for both of us.

You see, to be able to successfully pitch a full change to Proton to my wife, we need to be able to:
- create additional Address Books (of course wouldn't be complete without edit and delete, too)
- share address books with other users