buying a round trip ticket doesn't always save you any money.
This may have been the motivation many bought round trip ticket. But I've once found out there's particular advantage of round trip ticket. For a short trip that is, say a quick weekend trip to Snowbird, for example...
So on one Friday, I was sitting inside the plane which sat on the taxiway. But there's a storm outside. It took some time before they announced the airport is closed due to the storm, and deplaned everyone.
We all piled into the terminal trying to get re-booked for the next flight out. But the storm wasn't supposed to let up till next morning. And the only flight still have seats available wouldn't be till late that day...
Only problem was, I was supposed to fly back on Sunday evening. I would only get one day of skiing instead of 3. Naturally, I wished to cancel the trip. No problem, the whole round trip ticket was refunded. 30 seconds later, I dragged my skis out the terminal and drove up to VT for the weekend. I missed the weekend of skiing at Snowbird. But I wasn't missing any money from the flight.
Had I bought 2 separate tickets, especially if by a different airline, I'd have trouble getting that return leg ticket refunded.
Fly often enough, one eventually gone through different kind of "issues"...
One long time ago, I was at Aspen airport waiting for the puddle jumper to take me to Denver and back home in the east. But there's another storm and the puddle jumper couldn't land. So the bunch of strangers waiting there decided to rent a van together and drove to Denver. Of course, we missed our connecting flight out of Denver by that point. Those of us who bought a single ticket from the same airline for our Aspen to Denver to home (wherever that was) all got our Denver to home leg rebooked for free. I even got my Aspen to Denver leg of the ticket refunded. But those who bought their 2 legs separately had more trouble getting a new flight rebooked.