What is meant by "the Western side" in Surat Al-Qasas, and what is the reason for mentioning this particular side?

Ibn Kathir mentioned that the place where Allah revealed Himself to Moses was on the western side of Mount Tur, to his right. Ibn Juzayy stated that the verse: "And you were not on the western side" is addressed to the Prophet Muhammad, peace be upon him, and its purpose is to establish proof of his prophethood by informing him about the story of Moses. The western side is the place where Allah spoke to Moses. The reason for mentioning this is to affirm his message, peace be upon him, by virtue of his informing about past stories that he could not have known except through revelation, as in the verses: "And you were not with them when they cast their pens as to which of them should be the guardian of Mary. Nor were you with them when they disputed," and "Nor were you a dweller among the people of Madyan," and "And you were not on the side of the Mount when We called out, but [it was] a mercy from your Lord."