Though it was a different medium and a long time ago, I've had people steal my characters for their roleplays, graphics and faceclaims. I've had to report many people for use of mine and my friends' images and worse yet, when people just straight up drew fanart of my characters, they'd eventually just start claiming their appearances for their own ocs.
Watermarks at least decrease the chance of taking an image/character because it looks nice. But people can be assholes no matter what sometimes. It pays to take those measures nevertheless.