It's not about the storage of the value, it's the fact that PG will potentially change the value you provide compared to what's stored when using 'timestamptz'. If you use UTC and send it as UTC and request it as UTC then everything is fine but there are plenty of edge cases when using ORMs, handwritten SQL, different server timezones, or type conversions that can lead to issues on the way in and out of the database.
Having the database store anything other than what you provide is always a source of bugs.
Having the database store anything other than what you provide is always a source of bugs.