Without getting into too deep aeronautical engineering stuff, the tapered wingtips help reduce tip losses. A squared-off tip dumps (wastes) at lot of lift. Therefore, almost all real planes have either tapered tips or some sort of fence or device to reduce tip losses.
Area is area, regardless of the shape. A real aero engineer showed me an explanation of why with Spitfire-shaped wing, with its tips that taper to nothing, is the most effective use of wing area. But it's only slightly better than straight-tapered wings that approximate the ellipse.