user levels

Hi,
Pardon my ignorance, but what is the point of the various levels a user can attain? Bering the admin I am set at 10 and I have a friend who helps run the site who, when registering is now set to 9. However, setting at 9 seems to be no better than setting at 1. They cannot edit or delete posts but I would like to give them this function.
Is there a way to make it so that selected users have as much control as the admin?