In the void of emptiness, Om was the first non-empty object. For there to be anything, there needs to be an absence of it. In a wave, the peaks cannot exist without the troughs or the valley. Similarly for there to be anything there needs to be opposites, or rather a difference, a disagreement, whatever you may call it. Om embodies the first vibration, wave which began existence. But for there to be existence, there needed to be emptiness or the void. They are mutually existent, that is why the Om is the beginning, the duration, and the end, and yet ruled by the emptiness around it. As for why Om is helpful in meditation, it is said that it is the closest thing one can come to the beginning and hence everything. By reciting om and feeling the vibration go through your head, you are becoming one with existence.