So by adding hydroxide ions in the fifth step you balance the charge by adding electrons to one side. Remember, charge is concerned so whatever charge is lost must be gained. So that's why you need to balance the charge by adding electrons to one side of the reaction. Next, you multiply each half reaction by appropriate factor so that electrons gain equals electrons lost. Once again, this comes from the conservation of energy or the conservation of charge. In the final step, you basically add the two half reactions and you check to make sure that they are, in fact, balanced.