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.