No one is telling the user what he should use this for, there's plenty of situations where you might want to convert/encrypt/elaborate data without being launch codes for nuclear missiles and this seems like a pretty good tool, all browser based too, opposite of many other more famous tools that require communication with a server.
I am not talking about the actual encryption/decryption/data-wrangling stuff in this HTML page. All of this is obviously very neat and very usable.
The reason I do think this is a job ad is the fact that it's the GCHQ that is publishing it. Seriously, a spook agency is publishing neat open source stuff. I can only think of two reasons for this to happen and both align:
a) employee happiness (few people enjoy doing stuff in secrecy, I think)
b) using the by now well-established mechanics of corporate branding to make the GCHQ appealing to a larger amount of developers/hackers.
I think the latter is the dominant factor, and this is why I called this a job ad.
d)"Just-your-friendly-security-agency!"