Samigon Rubber Wide Angle Lens Hood is a screw-in collapsible rubber lens hood that is specifically designed to be used with wide-angle lenses. It can remain on the lens permanently, because it only lengthens the lens minutely in its collapsed state, requiring hardly any additional space in the gadget bag.
Lens hoods are primarily designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. In addition, since the end of the lens is extended, one also gets the added benefit of some extra protection from accidental impact.