"On Saturday 20 July 2013, in the basement of the Guardian's office in Kings Cross, London, watched by two GCHQ technicians, Guardian editors destroyed hard drives and memory cards on which encrypted files leaked by Edward Snowden had been stored."
GCHQ was merely observing to see that the Guardian staff were cooperating. Instructing journalists to remove their 'special storage chips' while leaving others intact would be a breach of operational security. This event was more of a PR move because GCHQ, the NSA, and others had no idea what was going on and had to be seen "doing something", even if it was pointless, absurd, and violated the free speech rights of the newspaper and journalists.