Ships don't always float on mirror flat oceans. Waves wave around them which can create more buoyant force one the ends than at the middle. This makes the middle "sag down" and the structure of the ship has to be strong enough to take the associated tension and compression. It can also happen that there is more buoyant force at the middle forcing it up while the ends are somewhat less supported. (Also known as "hogging".)

The longer the ship is the stronger these forces can be.