Head movement is one of the standard boxing skills and more advanced you are, the more you will need it. Your head can handle only limited number of punches and most of them will make you confused and disoriented so knowing the right way of head movement can save you from many hits.
The basic idea is simple: make yourself a moving target so it will be harder to hit you. Defending your head all the time is an impossible task and sooner or later you will fail and get a punch. By moving your head constantly, you make your opponent guess and wonder what your next step will be.
Head movement will also make your hands free. Your hands are not preoccupied with blocking and you can use them for counterpunch.
If you make your opponent miss a punch, it will take him a big amount of energy. He will become tired just from trying to predict your next move. Your opponents will be also unsettled or even upset and it will make them more vulnerable.
The better you become, the more you need good head movement skills. Your opponents will be faster, stronger and more experienced and without fast head movement you will have no chance to defeat them.