Every marathon training schedule should include a number of shorter races. Ideally, there should be a 5k early on, then maybe a 10k, and a half marathon about 4 weeks before your target race. It doesn't have to be exactly like that, though. If there are races within your vicinity of slightly different distances, then participating in them will still be beneficial. Taking part in a race will get you used to the atmosphere, and the organisation of a race in general. It might teach you how to drink from a cup, or how to pace yourself correctly. Most of all, a race is always a great workout. I don't know about you, but I can always run faster and at higher intensity if there is a number pinned to my chest.