After leaving Arafat equipped with knowledge, heart illumination, and a right vision, pilgrims see that true happiness lies entirely in obeying Allah, and that misery lies entirely in disobeying Allah. At this point, they see Satan’s enmity, and how he makes people fear poverty if they spend, he makes them fear other than Allah if they repent, while he is betraying them.
 Allah the All-Mighty says:

﴾And Shaitan (Satan) will say when the matter has been decided: "Verily, Allah promised you a promise of truth. And I too promised you, but I betrayed you. I had no authority over you except that I called you, so you responded to me. So blame me not, but blame yourselves. I cannot help you, nor can you help me. I deny your former act in associating me (Satan) as a partner with Allah (by obeying me in the life of the world). Verily, there is a painful torment for the Zalimun (polytheists and wrong-doers, etc.)."﴿

 Thus, a pilgrim expresses his enmity to Satan symbolically by throwing pebbles, so this throwing would be a physical expression and a pledge to take Satan as an enemy and reject his thoughts.
Imam Ghazali said: "You should know that when you are throwing pebbles in Aqaba, you are in fact throwing Satan with them and destroying him. This can only be done by obeying Allah's orders."