Q. rs.addArb() Error in MongoDB 5.0.x

I did rs.addArb().
but, the following error came out.

MongoServerError: Reconfig attempted to install a config that would change the implicit default write concern. Use the setDefaultRWConcern command to set a cluster-wide write concern and try the reconfig again.

So, I searched the Internet to find out the cause of the error and found the following answers.

Based on the answer, I was able to solve the problem by changing the Default Write Concert.

However, I found a contradiction in rs.add() where no errors appear.
I think this was a contradiction because I didn’t fully understand the answer.

Can I ask for a more detailed explanation of the answer?

I worked on the following scenario.

  1. rs.initiate( { _id: , members: [ { _id: 0, host: <hostname:port> } ] } )

  2. rs.add( <hostname:port> )

  3. rs.addArb( <hostname:port> )