I am no expert but it appears he had his own cult which was formed around him during his reign as king. He was said to be a devout Christian too. Miracles in the form of miraculous healings were said to have occurred after they opened his tomb. What I think it really boils down to is that he was canonized as a saint. Saints are holy, if you have a part or possession that once belonged to a saint it might bestow special holy blessings on the bearer or place? Wikipedia calls the practice Reliquary. Where they divide up body parts or bone fragments of saints, and send a piece of them off to different cities or churches. They would often be displayed for example on feast days or holy days. People would then come to venerate these relics (basically honouring the saint) by kissing or kneeling before it and bowing etc.