Let's think of a paper sheet. If we tried to stiffen it from its primary state, it couldn't support its own weight. However, if we bend it, the sheet achieves a new structural quality. The shells act in the same way. "You can't imagine a form that doesn't need a structure or a structure that doesn't have a form. Every form has a structure, and every structure has a form. Thus, you can't conceive a form without automatically conceiving a structure and vice versa".  The importance of the structural thought that culminates in the constructed object is then, taken by the relationship between form and structure. The shells arise from the association between concrete and steel and are structures whose continuous curved surfaces have a minimal thickness; thus they are widely used in roofs of large spans without intermediate supports.
In structural terms, they are efficient because they resist compression efforts and absorb at specific points on their surface, especially near the supports — small moments of flexion.