TimeZone parameter in postgresql.conf postgresql documentation says:


  All timezone-aware dates and times are stored internally in UTC. They
  are converted to local time in the zone specified by the timezone
  configuration parameter before being displayed to the client.


timezone parameter is a session parameter. When all TIMESTAMP WITH TIME ZONEs will be stored as UTC and the client connection will specify the session timezone, then what is the benefit of setting TimeZone parameter in postgresql.conf?
Like with any other parameter that can be defined for every session, the setting in postgresql.conf is the default in the absence of an explicit setting for the session.