Every dollar invested in inventory represents another dollar that has to be borrowed somewhere else. For states that have an inventory tax, the spare parts inventory is taxed on total value, not on the inventory items that are used in the daily operation. Each time a spare part is moved or handled during its time in the storeroom, there is a cost involved. Inventory management practices that do not review inventory regularly to identify obsolete items and target items over the maximum stocking levels also drive up the carrying cost.
Source: https://www.lce.com/Whats-the-real-cost-of-spare-parts-inventory-1189.html