Let’s say this out loud right at the start — no one actually wants to hold spare-parts inventory. Why would you? Inventory ties up working capital that we could otherwise use for more productive purposes.
But we need to hold this inventory. We need to be able to supply items in a timely manner — usually far more timely than the supply chain would allow. We also need to take advantage of purchasing and/or manufacturing efficiencies. So for economic reasons, we purchase more than we might immediately need, and hence we hold some inventory.