Some say its because its a form of pledging allegiance, some say its a form of respect to say he is above our heads, some say its because the Imam will touch the heads of his followers perfecting their intellect. Allah knows best.
As for your second question, please read the following book
https://www.al-islam.org/fazail-ul-mahdi-ali-akbar-talaafi/benefits-imam-mahdi-aj-during-his-ghaibat