There's no synergy units (thank god). There's 4 difficulties, and as usual the harder ones give more points per energy. The elite version is supposedly true end-game tuned (basically on Gilgamesh trial level), but Global always adjusts tuning, so it may be easier (or harder). Elite in JPN gave 7,000 currency. As the boss is killed serverwide, he gets more and more health added, and the base points get a multiplier, so by day 2 he may have twice the health pool, but award twice the points on a kill, for example. If you wipe, you still get some points based on how much damage you dealt (wiping at 50% gets you half points, etc).
You get 5 orbs that have their own pool / cooldown (1 hour refill) to use on the raid, so running the raid won't interfere with any of the other content you do in game, it's totally "free". When you reach X amount of points, you get a personal reward. In JP, Yshtola was given automatically after 600,000 points, Minfillia at 200,000 (but supposedly all 3 are very common from the raid summon gate). There's no global shared milestone rewards like christmas, but there are global ranking rewards. As usual, the ranking rewards are only for the dummy whales who spend $2000 in refreshes to get top 10 for ~100$ worth of lapis, so may as well ignore the ranking entirely.
You spend the points on a special raid gate that gives all 3 units, their evo materials, and various other stuff like crafting mats, consumables, etc. The FF14 units also have exclusive gear you can make from raid drops.