According to Aughra's description in World, she believed the crystal was a sort of nexus that occurred on multiple worlds (based on the arrival of the UrSkeks from their home realm via the crystal), but only the one on Thra gets an unusual volume of energy from the suns during conjunctions. As Gebirg said earlier, it's implied that the UrSkeks wanted to burn away their evil elements (which was the cause of their banishment from home, a "perfect" realm where no evil or imperfections were tolerated even mixed with good) and wound up splitting into the Skeksis and the urRu.
Hey, maybe that explains how Captain Picard got his own Shard of the Dark Crystal to use as a paperweight. Maybe that's how Armus from "Skin of Evil" was created -- his people used their own Crystal to separate out their evil side! It all fits!!
