> The timezone is implicit in the appointment, it's the timezone of the place you're making the appointment at.

That's not necessarily the case, though. The location is implicit (or explicit, I guess) in the appointment, but the timezone may have changed by the time the appointment arrives. Say for example, your region decides to stop doing daylight saving time changes. But it's just your region, not the entire existing time zone, so a new zone is created to reflect that.