It's probably not something most folks have to worry about. But for the sake of example, say that you download an image to be later used in a document/presenta

It's probably not something most folks have to worry about. But for the sake of example, say that you download an image to be later used in a document/presentation that'll be made publicly available. The image server might choose to encode information like retrieval time, IP address, account you're logged in with, and any other tracking info it might have through cookies in an invisible watermark. Then after you post the image as part of a document/presentation, the original source can make that connection. Ideally anyone tracking you would make the tracking mark cryptographically secure such that only they can interpret it, though there's also the possibility that they'd use some other mechanism that could reveal your personal information to anyone aware of the watermarking mechanism. Alternatively, say that the image is retrieved over an insecure line, e.g. through HTTP, and, say, some country (with loose notions of civil rights) wishes to track its propagation. Then they can intercept the original image, watermark it, and serve the malicious version to you. Which such a state might wish to do if you're, say, working on a technology that they're interested in.