Why is it not possible to connect remotely to the MySQL server without the bind=0.0.0.0 line?

Question

MySQL

Mysql-server mariadb is installed on the debian virtual machine, mysql-client is installed on the main computer.
The bridge network adapter is installed on the debian virtual machine to connect to the real network. Port 3306 is open, pings go to the server.
On the forums and in the MariaDB documentation, it is written that in /etc/mysql/my.cnf you need to remove the line bind-address = .
I didn’t have anything like that in my.cnf config by default, so there’s nothing to comment on.
Nothing helped until I came across on the forum that someone, instead of removing (commenting out) this line, changed it to bind-address = 0.0.0.0.
When I added this line, I reloaded the database, and lo and behold, everything worked!
Now my question is, why is this happening?