They were the means by which the accumulated evolutionary experience of ancient human groups — the specific adaptations they had developed over hundreds of thousands of years of living in specific environments — was transferred into the modern human genetic background.

The Neanderthals had lived in Eurasia for perhaps 300,000 years before modern humans arrived.

In that time, they had developed adaptations to the cold climates, the specific pathogens, and the specific resources of European and western Asian environments.

The modern humans who encountered Neanderthals, and who had children with them, inherited some of those adaptations — at a cost of whatever reproductive disadvantage the Neanderthal genetic background might have introduced, and with the benefit of whatever adaptive variants the Neanderthal contribution contained.

The result is that modern humans are not the product of a single, pure evolutionary lineage.