Inventory, whether it be produce, parts, food, or people ready to hire is waste. If you make or process more than you need it may seem wise but in reality, you’ve spent money on something you can use now. You may be able to use it next week or next year (if you’re not dealing in perishable things) but why would you pay in terms of labor, material or time to produce something you don’t yet need.
Generations past thought in terms of over-producing as “preparing for a rainy day” but in reality making more than you need or processing more than you can use in the near future is like putting money in your mattress to “keep it safe.”
The principle of lean is to produce or process only what you need today (or in a reasonable window) and no more. Anything beyond that is wasteful of your resources.