MongoDB Authentication Database Purpose - mongodb

Question

My question is about the mongo shell --authenticationDatabase option.
I was confused by it for a bit thinking that the the database I created the user in was the admin database since that is where the user is actually defined after reading this documentation here: https://docs.mongodb.com/manual/core/security-users/#user-management-interface
I understand I was wrong; the authentication database is the database the user has a role for. My question then is what is the purpose of the option "--authenicationDatabase?" See my scenarios below.
The command below will connect to the myapp database and also authenticate against the myapp database
mongo -u "bob" -p "bobspassword" --authenticationDatabase "myapp" myapp
The command below will connect to the myapp database and also authenticate against the myapp database by default. This seems to make the command above obsolete.
mongo -u "bob" -p "bobspassword" --authenticationDatabase "myapp" myapp
The command below will authenticate against the myapp database but not connect to it. Why? Is it simply to test credentials?
mongo -u "bob" -p "bobspassword" --authenticationDatabase "myapp"
This command would tell me "Error: Authentication failed." as well so I still wonder why I would need the previous command to find out my credentials are invalid.
mongo -u "bob" -p "bobspassword" myapp
Lastly, is there a situation where I need to authenticate against one database and connect to a different one like the command below? I can only connect to databases for which I have a role defined so why wouldn't I authenticate against the database I was trying to connect to?
mongo -u "bob" -p "bobspassword" --authenticationDatabase "myapp" myotherapp
Thank you for your insight!

Answer 1

In mongo you are connecting to a server not really to a database. You can change the database afterwards.
your code:
mongo -u "bob" -p "bobspassword" --authenticationDatabase "myapp"
will connect to mongod server and it will use a 'test' database if others not defined
The next one should work, since mongo takes the auth db from the connection string. Also it might be set different in a config file.
mongo -u "bob" -p "bobspassword" myapp
And final:
Lastly, is there a situation where I need to authenticate against one
database and connect to a different one..
No

Answer 2

When adding a user, you create the user in a specific database. This database is the authentication database for the user.
In most cases, you would want to create users in the admin database. This is a privileged database. However, you are allowed to create user in other databases. If two users have the same name but are created in different databases, they are two separate users. IMHO, this authentication database mechanism provides a sort of namespace so that users with same name are allowed (think about user management in a big organization).
But this is just how Mongo organize users and store then in authentication databases. The authentication database has nothing to do with to which databases this user has privilege (access).
Back to your case, you created a user in 'myapp' database and also grant it access to 'myapp' database. That's why you need to first authenticate against 'myapp', which serves as authentication database, and then connect to 'myapp' for accessing data.