TL;DR
Allow fulfilling queued ingredients in the crafting queue with ingredients obtained elsewhere.What ?
When I'm handcrafting, say, 200 inserters, the crafting queue would fill with building 200 gear wheels, 600 copper wire, 200 green circuits, and 200 inserters.Say, I know somewhere that has a cache of 500 green circuits, and 300 gear wheels, and I grab them, 200 green circuits, and 200 gear wheels from those grabbed those should instantly replace the 200 green circuits and 200 gear wheels in the player crafting queue, and the raw ingredients returned. I think it's natural to seek out the missing ingredients to speed up the crafting when you don't want to wait.