When I travel for business, things are straight forward: I usually land, do what I came to do and fly back home from the same airport. This makes no sense when I travel for pleasure. For instance, last year we visited Cartagena and then Bogotá. If we had to fly or drive back from Bogotá to Cartagena to then return home, we would have easily wasted a day. I understand that there might be instances when doing a traditional round trip makes sense if it is a lot cheaper and money matters more than time. However, if you want to enjoy more than one city, multi-city beats round trip most of the time.
Visiting Europe is one of the best examples. We arrived in London and returned from Milan. If we had returned to London the day before flying back home, we would have spent half a day, which is the time you have to allocate every time you fly. This includes the time it takes to get to the airport, plus the ~2 hours at the airport and flying time. We spent that half day enjoying Milano instead of flying back to London to then fly home.
When we visit Scandinavia, our plan is to fly to Stockholm, go to Copenhagen, then Oslo and return home from Oslo or Bergen instead of returning to Stockholm to fly back home. Even if the round trip ticket were significantly cheaper, doing the multi-city would makes more sense to us.