Running a half marathon or a full marathon is a long drawn race. Therefore, being able to maintain a constant pace throughout is one of the keys to completing the distance and achieving your goal timing.
Not every long distance races will have pacers to help keep the pace, you may not get to be flagged off with the pacers of your targeted timing or inaccurate pace readings may appear on your GPS watch when signal drops.
Having a pace band with you during the race can help you keep track of your pace, even in less than ideal situations.