Booking a round trip is usually more cost effective than booking two separate one-way flights. But booking cheap multi city flights for that same trip can sometimes be cheaper than flying roundtrip.
You could, for example, fly from New York to Paris, and then travel to London and fly back to New York from there. You’ll likely need to take a train or bus to get to London, but it could all add up to cheaper trip than if you had booked round trip to and from either two cities. And you’ll not only save some cash but create a unique travel experience.