Users want a feature that allows them to easily compare the prices of roundtrip tickets against two one-way tickets, as the pricing can vary significantly depending on flexibility with dates.
It used to be a given a round trip was cheaper than a pair of one ways, but that's definitely not the case any longer. If your dates are flexible, play around with dates as a one way until you find the combination of price and convenience you like \*then\* plug both those dates in as a roundtrip to see if the price changes. I just booked tickets to Montreal with American Airlines. Roundtrip premium for two was $883. The exact same trip booked as a pair of one ways was $753. The only thing you give up is if you have to cancel the trip or you miss the first trip, it doesn't automatically take care of the second trip. These tickets are fully refundable as trip credit so it's irrelevant here, and if you have travel insurance it's likely also irrelevant.