The price for a vehicle can easily change during negotiations. Features or warrantees can swing loans (up or down) by thousands. Calculating the cost-per-thousand dollars borrowed helps you quickly understand how those changes will impact your loan payments. Armed with that information you can better decide how to spend your money.
Of course a good place to start is with a budget estimate or an idea of loan payment in your price range. From there, you can use the Cost Per Thousand figure to deal with the unexpected.