Never understood this when booking car rentals.

If I want to rent a car, and rent it from Location A, and return it to Location A, the price is significantly cheaper as opposed to having a car rented from Location A, but returned to Location B.

Why is this? I can't find anything online that would help me understand this? When I am at car rental agencies, sometimes when we are waiting on a car, they explain it is coming from another branch into the branch I am it.  Then why is it so important that car rentals go back to the same place for a cheaper price?