Who will create purchase order and whay ??

It is done by the purchasing/procurement department, operations or finance, because they are responsible for purchase a new item 

Procurment Department , because they are the authorized departments to initiate the request for purchasing goods or services

In a company where task are not distributed, the responsibility is shouldered by the staff in-charge in the inventory because he knows the number of quantities to be purchase that can support the business operations for given period of time.

the purchase assistant will create the purchase order so that it confirms the seller wat a buyer wants which he s willing to sell.