Deflection calculations are used to determine how much a structural element, like a beam or a bridge, bends or deforms under load. Engineers use these calculations to ensure that structures can support the expected weight without excessive bending, which could lead to failure or safety issues. These calculations take into account factors such as the material properties, the shape of the element, and the type of load applied. By understanding deflection, engineers can design safer and more efficient structures, ensuring they meet building codes and performance standards.