The parts library has a massive value.

For a lot of parts the exact details aren't crucial. If I'm using 4k7 0603 I2C pullup resistors I'm more than happy to swap them for 5k6 0402 if they just so happen to have a reel of those lying around and it means not having to wait two days for a restock.

Same with plenty of other parts. Maybe 10% is crucial, the rest can relatively easily be swapped out with whatever happens to be available. Transistor from a different manufacturer, generic level converter with two extra channels, LDO in a different package? If I know what is available, for a proto run I'm more than happy to make a few small changes!