What is the difference between PO and GL? Why the companies use GL account instead of PO?

The PO is External Purchasing document for ordering goods purchase from Vendor.

When purchase orders are entered in the standard SAP system, the general ledger account that is posted to is derived in the below ways:(1)    If it is a purchase of inventory (stock), then the general ledger account assigned to the valuation class that is linked to the material is defaulted;(2)    If it is the purchase of a fixed asset, then the general ledger account that is assigned to the account determination that is linked to the asset class that the asset belongs to is defaulted;(3)    If you are not purchasing fixed assets or inventory, then the system will use the ‘Account Modification’ that is assigned to the ‘Account Assignment Category’ which is specified in the purchase order. This Account Modification is linked to a general ledger account in the account determination table (transaction OBYC). In this transaction, if you double-click on transaction key GBB and look at the relevant  Account Modification (called “General Modification” in this table), it is the general ledger account that does not have a valuation class assigned to it that will be defaulted in the purchase order.(4)    If you are not purchasing fixed assets or inventory but you do not want just one general ledger account to be defaulted for all purchase orders that use that account assignment category, then you can assign material groups (which are specified in the purchase order) to valuation classes, and in turn assign the valuation classes to general ledger accounts.