What you actually get by staying versus rushing back to Sydney.
A day trip covers Echo Point, Scenic World and one short walk comfortably, especially if you're on a guided tour handling the transport. It's the right call if the Blue Mountains are one stop among several on a short Sydney visit.
Staying one night opens up the Prince Henry Cliff Walk properly, an early-morning Echo Point visit before the tour buses arrive, and dinner in Leura's small but well-regarded food scene. Worth it if you have the extra day and want to actually walk rather than just look.