What exactly is the use of the column USER in a pg_hba.conf file?
"someone needs to be logged in as the user 'user123'" - no, that means that someone tries to connect with the Postgres database user user123.

This can be used to allow certain database users to connect only from certain client computers, e.g. the user for a web application is only allowed to connect from the IP of the webserver. 

Or this can be used to prevent superuser logins from remote connections by only allowing  local connections for the database user postgres.