A watermark on a photo is like a signature on a painting – the purpose is establish a link between the artwork and the creator.
It is best to add watermarks to images shared on social media where anyone can download or screenshot the photo and share it and re share it and this way the photo will loose it’s link to the original creator and no one will know who is the legal owner of the photo.
Usually photographs are owned by the photographer who created them and removing watermarks is illegal. It is also illegal to use photos without the permission of the creator.
In this video I am going over what the purpose of watermarks is and when it’s best to use them: