In business a photo can often make the difference between a sale or no sale, or more important, which shop the sale goes to. Some businesses, like fashion, are almost entirely "photo" based. Others, like food, cars, etc. rely on it extensively.
Here's an example in which, for arguments sake, we state that all non-photo factors are equal and the best they can be.
Your business spends $100,000 a year to have top quality photos taken for the products you sell and you find the photos lead to a five to ten fold improvement in sales vs. stock or no photos. Now say all your competitors just steal your images and use them for their sites. Your sales drop back to where they were and your advantage is gone. And now you've got to make up the $100,000 to top it all off.
Having a watermark won't stop lesser competitors from sealing your image, but other "on par" competitors will likely not due to potential litigation. If you don't have a watermark everyone steals it with the line, "I didn't know it was something I couldn't use ... my web guys just found it on the internet. I'll talk with them." (or whatever) So now you have to constantly police the internet for competitors using your images, deal with take-downs, and lawyers.
Hope I've cleared things up a bit.