It demolishes 'population replacement' theories - where waves of invaders wipe out the earlier inhabitants. Most Britons are descended from the people that arrived at the end of the last ice age, and DNA indicates they came up from the Basque Country and the area around Toulouse and Carcassonne as it happens.
Also reckons that Germanic languages have been spoken in Britain since long before the Romans arrived - it was Celtic romanticism which came up with the idea that Britain was entirely Celtic speaking. It was language, goods and technology which travelled, not nations.