Sometimes when a item drops it can be difficult to tell if it’s an upgrade over your currently equipped item. Especially if the secondary stats are different. Maybe haste is your best stat, and is on your current gear, and the new item has crit, which isn’t as good as haste, but the item also has more main stats. How do you tell if it is really an upgrade?
Luckily, there is a way to mathematically calculate if this new item is a true upgrade over you current gear i.e. increase your throughput. It is done using stat weights and some simple multiplication and addition.