Lens hoods are primarily designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. Another benefit of the lens' extension is some additional protection from accidental impact.
The Kalt 72mm Screw-In Rubber Lens Hood folds down and 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. It is designed to match the focal length of standard lenses (50mm). Double threaded to accept filters.