alt Hacker NewsA number of prominent corporations used to embed these in the background images of their internal webpages, so that leakers could be identified from the screenshots they shared. Caused a whole fun adversarial loop where journalists had to transcribed and/or redraw screenshots before publishing to avoid exposing the identity of leakers...
Watermarks are often just another form of DRM.
Digital Rights Management isn't just about restricting what you can do with the content, which is often futile anyway. Another way companies can manage their rights is by making sure pirates are properly identified and caught.