Round Ticketcircular ticket
"What's the roundtrip or "'What's the return"?
When you say a round trip in the U.S., it means that you are returning a second times and are not making a round ticket from XXX to YYYY", i.e. "a round ticket from XXX to YYYY" in AE and "a round ticket from XXX to YYYY" in BE? It is not true - I see from your hyperlink that Mr Al is a Turk and not a mother tongue of English.
Must have thought that the plurals for AE One-Way-Ticket (BE Single) are two-sided! Reverse ticket (BE) and round ticket (AE) are the right choices. Brisbane to New Zealand is a homeward voyage. Brisbane to Adelaide to Melbourne to NZ is a round tour. This would be referred to as a multi-stop in AE.
Travelling by plane - What is the aim of a return ticket?
Particularly on longer (non-regional) journeys, a round-trip ticket usually cost significantly less than two single ticket prices (except reward ticket purchases with points/miles), especially on longer (non-regional) journeys. I've come across many cases where a single ticket was actually more costly than the total round-trip on the same journey for a single ticket. Occasionally it still is, but today a return ticket cost exactly the same as the total of the two one-way rates for home travel in the USA, as you have noted.
Inland /regional services outside North America are following more and more the same pattern where a return fare is calculated on the basis of the total of the return segment, but outside North America it is not yet as widespread as inside North America. Generally, if your return journey is so much late that it would be for nothing (e.g. you had a busy office meet or convention and the late arrival has already made you miss it), the contract of carriage allows you to generally make a full cancellation for a full refund. However, if you are unable to make a full cancellation, you may be entitled to a full cancellation.
If you have a round-trip ticket, the whole ticket, plus any non-retarded returns, will be canceled and reimbursed. For two one-way fares, the carrier is not obliged to reimburse the cost of the flight (although sometimes they do, especially if it is the same carrier and more likely if you are a frequent flyer).
The purchase of a return ticket can therefore offer some degree of security against flights being delayed or cancelled. If something occurs that causes you to modify your itinerary, the airline's modification charges are usually calculated per ticket, not per segments. So if you have to postpone your entire journey (round trip) by one full stop, you only have to cover the exchange charge once with one round ticket and twice with two round trips.
But on the other hand, as you have already noticed, the modification charge sometimes cost more than a single ticket and you only need to modify your timetable for one part. Actually, in this case it is actually less expensive to make the changes with the two one-way ticket by just canceling the ticket to be changed and book a new ticket for this part.
That is much more likely on relatively shorter inland journeys where tariffs are already very low. This is less likely for longer journeys where the one-way fare is likely to be higher than the modification fee.