Bridge fittings are used to secure containers together that are side by side.  

Bridge fittings would be useful, for example when cutting the walls out of two containers to make a large workshop.  The bridge fittings would prevent the containers from shiftiing and would allow for the sealing of the roof joint of the two containers.  They do so by securing the corner cubes together by turning the center bolt with a wrench.  You then secure a nut against one of the hooks to prevent the fitting from loosening.

Bridge fittings are NOT to be used for securing two stacked containers together as the hooks don’t close enough to secure them even when tightened all the way.  They are only to be used for containers that are side by side.