Usually, patents of a lens are for the optical design or formula. There are a almost unlimited number of optical formulas, but only a relative few that are practical to build and give high quality. Before computers, designers would lay out a lens design drawn to scale and trace the rays of light as they passed thru each element. As you can imagine, it was very difficult, and expensive to search for a better design.
Now computers do the ray tracing, and even so, its difficult, and expensive. So, when a practical formula is found, its patented. This prevents someone else from using the formula that you spent so much money finding.
Patents can, and are sold or licensed, so even if a company doesn't use the patent, it had value.
Take that nikon 14-24mm zoom. That optical formula is patented, so anyone caught copying it would be quickly shut down by Nikon's attorneys. It could very well take years of searching optical formulas to find something equal or better, its not something a engineer just sits down and sketches overnight. There are undoubtedly ones that cost a fortune to manufacture, but something practical and good is rarely found.