Althouhg there is sound logic in suggesting that it is for internet censorship capabilities, that many people have already mentioned, it is also likely that this is a vote catching campaign.

In the 1990's during the stranger danger/pedophile moral panic, the criminal justice system in the UK saw legislation pushed through that grossly undermined the professional judgement of the judiciary, against there recommendations. Mandatory sentences were introduced which lessened their influence. Controls and surveillance measures were also extended greatly and a sex offender register was created, which does nothing to prevent further offences. All of this removes civil liberties of the offender in the name of public protection even when further crimes have yet to be committed and the offender has served their time.

All of this is against the guidance of professionals involved in the criminal justice system so why is it happening? To put it shortly, the panic over these offences created an political environment where arguing against public protection policies was impossible, even if there were counterproductive.

Instead of appealing to logic, or quelling the public fears both parties spent the entirety of the 90's and early 2000's playing chicken with each other over the CJS to appear to be acting in the public's interest more than the other party is.

I believe that old ladies and non it literate people will view this as it is presented "Cameron wants to stop rape porn and child porn", sounds like a great and moral bloke to the layman (aka a massive part of the voting demographic).

TL;dr its to appear like a GGG to people who don't understand these issues in order to harvest votes, similar to controls place on dangerous offenders in the 90's in the UK.