Spying is a big problem in Eve. At the most basic level a spy is able to take screenshots and copy/paste text to send back to the entity they're spying for. By watermarking both text and forum backgrounds the data effectively becomes tainted in the sense that the screenshot/text will have unique characteristics that allow the original poster to identify who copied the data in the first place.
The whole flow would be:
- X is a player in alliance FOO but is actually an agent planted there by alliance BAR.
- Y is a player in alliance BAR but is actually an agent planted there by alliance FOO (or any other).
- X copies intel from FOO's forums and sends it back to the people in charge of the spy program in BAR.
- That intel gets shared with key personnel in alliance BAR, so they can take action based on the gathered intelligence; unknown to BAR, among that personnel is Y
- Y sees that intel and sends it back to their leadership in alliance FOO
- The people at alliance FOO identify the unique data on that intel and track it down to X and proceed to kick him for being a spy.
As someone who is involved with alliance leadership stuffs: This may sound convoluted but it's really bread-and-butter level stuff in Eve, it can get significantly weirder. For example this article discusses whitespace character-based fingerprinting: that's amateur level, like described elsewhere in this thread.
To answer your question: counter-intelligence. If your method for tagging data is known and easy to replicate, such as watermarking your forum userID in screenshots, the people at alliance BAR can edit the screenshot (or forge a new one) where they insert the userID of an innocent person in alliance FOO in the gathered intel, this way when Y grabs the data for sending back to FOO, they'll be unknowingly sending evidence that incriminates a loyal member.
edit: I noticed I didn't address your last question. Signing data is just regular cryptographic signing yeah, in the above example to prevent tampering you'd insert the hash from the userID plus a secret salt for example. You just need some way to prevent the hostiles from incriminating someone else.