The pack qty feature is used to indicate that you must buy a product to a supplier with a quantity being a multiple of X.

For instance, if you buy bottles of wine, the supplier may require that you purchase the bottles by box of 6.

In such case, when you will create a purchase order with this product, the system will ask you how many boxes you want to purchase. However, when you will receive the products, this is the unit quantity (the bottle) that will be put in stock by multiple of 6.

If you want to predefined the pack qty, you can do it from the product view, in tab "Suppliers" : 

When you add a product to a PO, if a pack qty is configured, it will be used by default, however you can manually change it.

In the screenshot below, we order 5 boxes of 6 bottles (30 bottles in total) : 

When you will receive the purchase order, you will do the reception for the boxes. In the example below, we receive 2 boxes of 6 bottles :