Why does the CBCDG combat the counterfeiting of currency?

The counterfeiting of currency is a crime. Counterfeit banknotes have no value.

While the overall economic losses to society are generally limited, the victims who suffer the most harm are individuals and businesses that receive counterfeit currency, because no one reimburses those who accept counterfeit notes. Counterfeiting currency can also undermine confidence in the payment system, making the public uncertain about accepting cash for transactions.   
Central banks deter counterfeiting by designing and issuing well-protected banknotes, and educating the public about them, to limit the losses that individuals and businesses can suffer from accepting counterfeit currency.