One of the most integral parts of the ship is its bottom structure. It is designed not only to give hull the required strength to withstand the weight of the cargo, but also to withstand the external hydrostatic loads that act on the bottom of the hull.
A ship’s hull is basically made up of bent plates welded together. If these plates are not stiffened, the bending moments on the plates due to the loads may exceed the value of stress that can be withstood by the material, and hence cause failure. So, the plates are stiffened (or their section modulus is increased) by adding stiffeners to them.