People use watermarks for many reasons. The most common reason is to stop thieves on the web from using your photograph and either marketing it as their own, or using it for monetary gain. With a watermark, people can still use your photo, but it will have a mark stating that it’s yours, rather than allowing them to claim it as their own work.
Watermarks can also be used to mark your photo as a “sample” on a webpage so that people know that is not the actual size, color, or photograph that they will be ordering. This is common when people have a lot of products they offer. It’s much easier to mark one photo as a sample than to watermark each photo of each product individually.
For more information on watermarks, what information you should include in your watermark and where within your image you should place your watermark, please read Visible Watermarks and Your Photos – The Benefits of Watermarking Your Images and What You Should Include