It is intended to prevent an incestuous relationship developing between the enablers and the operators so that capability is based on the requirements of warfare development, not vice-versa. (Interesting question - Does technology govern one's choice of capability or should one's requirement govern technology? Some years ago, I advised the then incoming Director of the Maritime Warfare Centre that we should exploit emerging technology but not be slaves to it... if it ain't broke, etc.)
Here's chapter and verse from the Levene Report into Defence Reform - An independent report into the structure and management of the Ministry of Defence dated June 2011:
Yeah I read the Levene report and disagreed with much of it. Basically, it just draws all the command with the name "Joint" altogether and says ok we give you a four-star to control all of you. But basically, its more budget controlling than producing a dedicated force. The US itself has cancelled their Joint Forces Command and they are much larger in many areas than the UK's military.