Read TFA -- Guardian decided to destroy the materials in order to protect its journalists. They were afraid that recovered computers would contain information about which Guardian staff accessed the machines and when, and that could be used as evidence against them.
The material was destroyed in this way because the alternative was to hand it all over to the UK government agency mentioned.
...what did the security services expect to achieve here?
To stop or severely impair The Guardian's ability to report classified materials.
The usual interpretation has been that UKgov was clueless about technology, refusing to recognize that there were other copies elsewhere. But maybe they knew that it was only "security theatre" and the intent was to only make threatening gestures without really affecting the situation.
It's still a threat of something worse in future, but in comparison with the way things have been going in western/northern countries recently, it was a pulled punch, a kind of ritual or "CYA" move.
I'd see it more like UK gov not caring if people think they're clueless - or even encoraging that opinion - so that people become sloppy with operating procedures or become less paranoid.
Being selective about which laws you enforce is worse than not enforcing any of them. In this case, the Government, through GCHQ is guilty of systematically violating its own draconian law (RIPA) that was designed to legalise all sorts of Orwellian bullshit.
