The Stock Purchases account is used to track the stock value of your purchased goods. The stock you purchase is deducted from the purchases account and moved to the Stock on Hand account.
The Purchases account is used to record all Purchase Orders you create in QuickBooks Commerce. It will give you an accurate and up-to-date record of all stock movements. This also allows you to know the value of your stock-on-hand. And at the end of the financial year, the Stock on Hand account will be offset by the purchase expenses.