Design patents are an integral part of the intellectual property strategy of many watch companies. And design patents are exactly what they sound like: patents on the design of an object.
Typical patents, called "Utility Patents" in the business, protect only the useful, or functional, elements of a product. So, for many brands like Rolex, Omega, and Lange, they file patents on their innovations in the materials that make their cases, their co-axial escapements, or new and wondrous complications.
But a large number of watch companies don't innovate in technology. They innovate in design and use off-the-shelf movements.
Companies like Baume & Mercier have their entire patent portfolios (near as I can tell) dedicated to design patents: over a dozen design patents in the US dedicated to the design of watches, watch faces, and watch parts.
Design patents only last 14 years from the date the design patent is granted, and only protect the look of the item, not the way it works.