In some narrations, he said: Do you know why Haji must go to Arafat and perform Tawaf when he is wearing Ihram? It is because Arafat is outside the border of the Shrine, and if someone is a guest of God, he must first go outside the gate, supplicate, lament as much as to become eligible to enter the Shrine, and only then enter [into the Sacred Shrine]. (Specifically, on the day of Arafa which) has a special supplication, while supplication is one of the prominent virtues and important duties of the day of Arafa in the land of Arafat. Likewise, if someone was not present in Arafat, but was in other places, one of the outstanding virtues of that day [for them to do] is invocation, especially the supplication of Arafa of the master of the martyrs, Hussain ibn Ali bin Abi Talib (peace be upon them). According to the statement that came from the Commander of the believers (peace be upon him): The pilgrim should keep supplicating outside the Sacred Shrine and purify himself until he/she becomes worthy of entering the border of the Shrine. Therefore, it is enjoined [By God] that the pilgrim should go out to Arafat, that is, outside the gate, before Circumambulation. For example, if a guest [servant] intends to visit his master, he must first stand at the door until he is allowed to enter, and then enter [the master's house]. Pilgrims who go to the land of Arafat on the ninth day, supplicate there in order to acquire eligibility. In fact, they beseech God to get permission of entering God's House. When they prayed, asked from the Almighty God [for permission] and became eligible, only then they enter the Shrine.
Hajj secrets, Session 3.