It sounds like you gain a mote by completing a level of XP after 20. It has been suggested that perhaps the gear that you get with motes of light, is to help you get further in raid and nightfall activities. Course not exactly sure what those both contain. Then you will kick a boss' butt and get better gear to replace/accompany your mote gear from the speaker. But alas this is still merely speculation. We'll start figuring it out in less than 2 weeks:)