Question
What is the significance of Bel leaves in Shiva worship?
Answer
The reason that the leaves of the Bilva tree are used in the worship of Shiva is that the Bilva tree emerged from the sweat of Shiva's wife Parvati, as described in the this excerpt from the Nagara Khanda of the Skanda Purana:
The splendid Daughter of the Mountain; the goddess [Parvati] became tired due to her sports. Drops of sweat appeared on her forehead ... and a drop fell on the ground. It became a great tree on the excellent mountain Mandara.... Parvati said, "Since this tree pierced through the ground and rose up near me, let it be named Bilva. If anyone approaches this tree devoutly and collects the leaves, he shall certainly become a king of the earth. If anyone with perfect faith worships me with its leaves, he will realize whatever desire he may cherish. If a person, after seeing the leaves of Bilva, at least maintains faith in the rite of adoration, I shall undoubtedly be the bestowed of wealth on him. If anyone makes his mind to eat the tip of the leaves, thousands of his sins perish automatically. If a man places the tip of the leaf on his head, no torture of Yama will give him pain." ...
This Bilva tree is very excellent, sacred and destructive of sins. There is no doubt about this that Goddess Girija resides at its root. It is remembered that Dakshayani is present on its stem, Maheshwari on its branches, Goddess Parvati on its leaves and Katyayani on its fruit. Gauri has been mentioned as present on its bark, Aparna at the middle of the bark, Durga in the flower and Uma in the branches and twigs. At the behest of Girija, nine crores of Shaktis have stationed themselves on its thorns for the sake of protecting living beings.
Those are all names of different forms of the goddess Parvati. Since every part of the Bilva tree is suffused with some form of Parvati, it is the ideal article to be used in worship of Shiva, just as Tulasi is ideal for worshipped Vishnu as it's associated with Vishnu's wife Lakshmi.
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