Segregation of duties is an essential part of internal control. It dictates that all accounting functions should not be completed by one person.
If the person who wrote the checks also makes out purchase orders, they could make out a fraudulent purchase order to themselves or a relative. The books might show a payout for computer repair services when in fact no computers were broken, the accounting clerk just took the money for himself. If one person made the purchase order and a second person wrote the check it would be much harder to steal.