Here's my preliminary list of potential reasons for faking a GCHQ slide. Not meant to be comprehensive, and not every possibility is highly likely. Some are included only for completeness, but I'm sure there are possibilities I missed. 

There have been a few incidents already where the slides have been challenged or at least partially debunked. The German prosecutor found a lack of evidence that the NSA was illegally monitoring their government and concluded that the alleged NSA order was a fake. Snowden has also accused The Independent of attributing slides to him that he never gave to them.

The why depends on the who. Candidates include:

GCHQ
Snowden (included for completeness, but least likely. Why fake this particular slide among many apparently legitimate ones?)
Russian chekist agencies
Chinese intelligence agencies
The Intercept
Intel service X

If the GCHQ, then there are a few possible reasons:
Part of a proposal/demo
Disinfo to discourage visitors from going to Cryptome
Disinfo to create paranoia
Disinfo to discredit other slides (Snowden suggests something like that here http://www.theguardian.com/commentisfree/2013/aug/23/uk-government-independent-military-base )
Disinfo to convince a FIS (foreign intel service) that their capabilities are better/more complete than they are (or were better than they were at the time)
If Snowden: 
???
Makes no sense as an isolated incident, little evidence to the contrary
Would not necessarily recognize a fake slide, since he didn't examine all of the ones he passed on
If Russian disinfo:
Discredit the GCHQ
Discredit western governments and undermine their intelligence agencies
Inflate the importance of Cryptome prior to attempting to use them to release disinfo
If Chinese disinfo:
Smear British cyber operations as part of their ongoing cyberwar with the west and related cognitive warfare
Discredit the GCHQ
Discredit western governments and undermine their intelligence agencies
Inflate the importance of Cryptome prior to attempting to use them to release disinfo
If The Intercept:
Fame
Money from being paid to publish a fake by any interested party
Financial compensation from media attention as suggested by Razer/Rayzer@riseup.net 
If Intel service x: 
Any of the above

Juan juan.g71@gmail.com 
Sat Oct 10 17:31:29 EDT 2015
>I still don't get who would make such a fake and why.