Say you’re using machinery for which you occasionally need spare parts to keep the equipment up and running. It is one thing if you buy the spare parts as they’re needed (they’re expensed in such a situation as they’re bought), however what if you have stocked on some of the parts since they’re either harder to get or the shipment period is too long for the production / usage of the machine to be halted.

Those spare parts may “move” that is they’re used very rarely and only if your equipment breaks down. Should those items be accounted as slow moving stock and valued to zero in your inventory? It is one way to look at the matter, however there’s also another option. 

The other option for treating the spare parts on your financial statements would be to depreciate them into expenses over a period of time, say 3 years. The period as such is entirely up to you, but it should preferably reflect the period during which you consider the need to replace some parts to be most accurate. If the period cannot be determined, it could range around 3-5 years. It should not exceed the period the asset, the equipment itself is expected to be in use.   

Yes, they’re part of your inventory and you’re depreciating them to expenses as the period passes. It’s not to say that the parts are part of your PPE now, but to merely suggest reflecting the expense of the item over the period the original part you’ll most probably soon replace, is being used in the equipment itself. It’s to reflect the expense essentially within and over the period it’s related to.