Hi,
The purpose of purchase UoM is to use diffierent measure from stock unit.
For example, in stock you count your item as bottle.
But when you purchase, you always purchase them by box.
In this situation, you can defined a purchase UoM for box which maybe equal to 12 bottles. Once you purchase 1 box, stock will be increased by 12.
Of course, expense which need to be distributed is based on inventory quantity.
You can change distribution method to by weighy or by volume