1. NDAs - if someone's in a closed beta, and starts posting screenshots, they can quickly identify the culprit.
2. Hacks - if someone anonymously boasts about finding some exploit in the game, and shows screenshots, they can be tracked down.
3. Abuse prevention - if someone posts screenshots of themselves abusing another player, or breaking the TOS in some other way - but with names blurred out - it would still be possible to find out who it was.
If a user emails support, and their email address is not directly traceable to their login(for example, if they use firstlast@gmail.com for battle.net instead of first.last@gmail.com as the sending email), it allows support to add that to the ticket.
It's not uncommon for griefers or cheaters to anonymously brag about their exploits via screenshot. If the screenshot were watermarked, identifying their account and whatnot, then Blizzard could take action against them.
The suggested idea (regardless of whether or not it's plausible) is NDA leak tracking: finding the people in private betas that are leaking information when they shouldn't be.
If it turns out to be true, it's a pretty cool yet creepy application of steganography in the wild.
"This item looks like it turned inside out, here's a screenshot!" (made up issue) -> now the support has all information they actually care about and which user may not even remember anymore - game version, time, servers, character ids, etc.
