A unit-of-measure conversion is needed whenever you work with multiple units of measure. For example, if you purchased an item in cases (meaning that your purchase order stated a number of cases rather than a number of pieces) and then stocked the item in eaches, you would require a conversion to allow your system to calculate how many eaches are represented by a quantity of cases. This way, when you received the cases, your system would automatically convert the case quantity into an each quantity.