Help protect the rear of your lens from debris and damage when not attached to the camera.
Camera lenses should always be protected when not in use. Buying lens caps are so much more economical than buying a new lens just because you may scratch it. Lens caps are built to protect the lens from damage.
They're made to block bright light, like sunlight, from hitting the front element and causing flare. But they can also help protect the lens if you're shooting in rain. What is the lens hood, and what is it used for? It helps protect the lens from bumps and scratches, it also shades the lens from glare.