Failure to take Britain out of the war had incredible strategic implications for the Germans. A country with an empire, a massive navy and plenty of space to build bomber bases remained in the position to threaten their gains in the west.
With England (Britain) out of the war you can assume that the USA would not get involved, and if they would where would their staging area be? New York Harbor is a long way from Europe and Bombers couldn't fly that far. Not having a bomber threat meant that Germans could use the men (and woman) working the FLAK guns for something else.
It also meant that the Germans now had to start building the Atlantic Wall to defend their vulnerable coastline. The time, money and materials invested in it could then have been used elsewhere.
Invading Russia without taking out Britain was the turning point for me.