The replication user should also not be the admin user, but a user who can only see content that is supposed to be replicated. The replication user is used to collect the content to be replicated on the author system before it is sent to the publisher.
But after removal of this user on author and publish instances I don't see any changes (replication is working).
Since for security reasons, we are not supposed to use admin user, we can choose to use this user. To use this user, you first need to login as admin and change this user's password and then replace admin with this user in the agent. By default, this user is not used.