Conversation so far:
What role do priests play in Mandaeism?
There are three grades of priesthood in Mandaeism.
What are the three grades of priesthood?
the tarmidia "disciples" (Neo-Mandaic tarmidana), the ganzibria "treasurers" (from Old Persian ganza-bara "id.," Neo-Mandaic ganzebrana) and the risamma "leader of the people.
How does one become a priest?
CANNOTANSWER.
What are some differences between priests and laymen?
There is a strict division between Mandaean laity and the priests.

Question: Why is there division?
Self-contained rewrite: Why is there division between Mandaean laity and the priests?