EETimes publishes project manager at Corephotonics Roy Fridman's opinion on the reasons why dual camera in smartphones might have an advantage:
By using such dual cameras, smartphone manufacturers are able to support extremely advanced imaging features while keeping the solution slim (below 5mm height), lightweight and robust.
When fusing the B/W image with the color image the result is significantly enhanced, even in extreme low light conditions.
Dual camera zoom uses a combination of a wide and tele lenses in order to achieve real optical zoom, similar to the electromechanical zoom used in the professional DSLR cameras.
Another major advantage of having dual cameras is the ability to sense depth.