Not all businesses sell a physical product, but many businesses have inventory that is made up of materials used as a part of a service provided to a customer. In either case, these products and materials need to be accounted for in the warehouse, when they are moved to job sites, and when they are used for a client job or within the production of a larger product.
As costs vary, the way you value your inventory can impact both your tax bill and how healthy your company looks to.
Product quality and customer service won't matter if you can't get inventory in the right place at the right time..