Permission of parents to be a Monk
This rule is put in place because joining a monastic order (like any thing else one does in life) is a karmic act, and one should strive not to do harm. The idea that a child might disappear into a monastic order, never accomplishing any of the 'normal' things that parents expect of children — gainful employment, grandchildren, social status or respect, or any of the other hallmarks of worldly 'success' — can be frightening and disturbing to a parent. One is asked to seek parent's permission so that one does not enter monastic life leaving a cloud of fear and resentment in one's wake. It's an act of compassion for parents and the world they live in.