You may need to record several different prices for the same product (Stock Item). For example: purchase price, sale price for single unit, sale price for multiple units, sale price for staff, sale price (and sales tax rate) for export sales, sale price (and sales tax rate) for domestic sales etc.
If you only have a couple of prices for each product, it is sufficient to enter this detail in the Stock Items spreadsheet. However if you have a more complex pricing system, i.e. you need to enter several different prices (and/ or sales tax rates) for the same product, it is easier to do this here.