Cost is a separate field because the cost of your Inventory may not be the same as the current price from the Vendor.

Check out https://www.odoo.com/documentation/functional/valuation.html

If you have a standard cost, you will put in there what the standard value is of each item in Inventory

If you have an average cost, Odoo will adjust the cost up and down automatically based on each receipt of new products.

If you have a real cost, Odoo will keep track of deliveries and adjust the price to reflect the cost of the last item shipped.