As much as I hate to admit it, it's to shoehorn people into running Alex and 24-man dungeons to get their gear. There are people who don't want to invest the time and skill into statics and random DFs to try to get good enough to clear them, so the relic grinds are for them to get their 230s and 240s. Also, by having people grind so many dailies, it keeps SE's content relevant by having people continuously run the dungeons. THAT is the real reason why you have to grind.