The sun can both outshine the brightest screen, and blind the photographer's eyes. Using a viewfinder avoids both those issues. The alternatives in strong sunl

The sun can both outshine the brightest screen, and blind the photographer's eyes. Using a viewfinder avoids both those issues. The alternatives in strong sunlight would be a rigid hood around the screen (which exist as accessories), or a fabric hood covering the screen and photographer (like what early photographers used).