Searching for people from scratch in reaction to a sudden demand will have much more latency than having someone you can pull from a recent roster of validated candidates.

It's exactly the same like how an integrated circuit can pull a sudden power demand from a capacitor placed next to it (often required by the datasheet), rather than from the power supply upstream, so there is no voltage sag affecting it and nearby components.

Or, in computing, prefetch and speculative execution, and such.

If you have a "sudden demand" demand I would say that you have other issues in your HR planning strategy.

You should not plan to do projects that require you to hire a special resource tomorrow.

And also: Having an open pipeline really does not hedge for that, and you open pipeline might not align with your sudden need.

It can take months to hire a specialist from both but if you're constantly spinning plates, you have some lukewarm contacts as well as a list of obvious no's.