Why WIP instead of Expenses in P&L

In production and supply-chain management, the term work-in-progress (WIP) describes partially finished goods awaiting completion. WIP refers to the raw materials, labor, and overhead costs incurred for products that are at various stages of the production process. WIP is a component of the inventory asset account on the balance sheet. These costs are subsequently transferred to the finished goods account and eventually to the cost of sales. WIPs are one of the components of a company's balance sheet. The WIP figure reflects only the value of those products in some intermediate production stages. This excludes the value of raw materials not yet incorporated into an item for sale. The WIP figure also excludes the value of finished products being held as inventory in anticipation of future sales.