This is a straight C&P from a site (link below) I have seen similar explanations before.
//By far more defendants plead guilty than not guilty, and there are benefits to doing so.
The defendant will have had a disclosure of evidence from the prosecution before the trial, so he will know what evidence they have. He will also more than likely have a solicitor or barrister who will advise him if a guilty plea is better than a not guilty plea.
The benefit of a guilty plea for the victims and witnesses are that they don't have to attend court or be cross-examined at all, so a guilty plea can save these people a harrowing experience. It also saves the court time and money, money which will have come from the public purse.
This saving of time and money by confessing guilt is often rewarded. In English courts a defendant who pleads guilty can expect a sentence more lenient than if he had pleaded guilty, up to a third reduction. The attraction is obvious. If you get a custodial sentence it will be shorter. If you get a fine it will be less.
The length of sentence can be important in the future. A short sentence may be considered "spent" in a few years. A longer one may never be considered spent and could hang over you and affect your employment chances for life.//
https://uk.answers.yahoo.com/question/index?qid=20120131003713AAN0Bns
I presume he has admitted to entering the house, but is denying the setting fire to the furniture? That may be part of the reason - he needs to discuss this fully with his lawyer.