SPPO catalyzes the reaction of polyphenols into two other structures, as shown above. In order to catalyze these reactions oxygen should be present as well! The quinones which are formed in such a reaction are very reactive and they will react on into brown and black pigments. These reactions are very complex, just as is the case for caramelization!
After the catalysis of these first two reactions however, the role of PPO is over. The reactions will take place by itself. However, without this PPO those first reactions will not occur, they will require to much energy to be initiated.