Purchased parts, sometimes referred to as raw materials, usually serve as corner stones in bigger puzzles. Similarly to any other project, without placing the corner stone the finished outcome would never exist, which is of magnitude if the project is a 5M$ space shuttle and the "corner stone" is a 20$ valve.

Evidently, it is imperative to ensure purchased parts availability. Nevertheless there are a couple of challenges for achieving it. Availability by definition goes hand to hand with maintaining stock, if the case is not a  1$ screw, but, a 50K OEM computer, you'd probably wouldn't like to keep too many lying around. On the other hand, key elements that govern your availability, such as, suppliers delivery time, suppliers availability and even obsolescence are uncertain, multiply it over numerous purchased parts and you're facing a dicey situation.