No province on the map borders as many others as does the North Sea. The North Sea borders an amazing eleven provinces. Six of the eleven are supply centers. No other province compares to the North Sea.
If you manage to force a fleet into the North Sea, your opponents will have to accept that you are going to move it more or less where you want to move it. There are so many choices; they can't block them all. A fleet in the North Sea is usually destined to capture one of the six adjacent supply centers, either by itself or with a convoyed army.
The North Sea is the single most useful convoy route on the map.
Doug Massey's research reveals that, in an average game of
Diplomacy, a fleet floats in the North Sea 83% of the time.
North Sea: The North Sea touches a whopping 11 other provinces, six of which are supply centers and another four of which are seas. England would be crazy not to take the North Sea in 1901 with either of its two fleets. England must maintain control of the North Sea because it borders two of his supply centers.
If Germany, or worse yet, France gets into the North Sea, London will soon fall because France can easily take the English Channel and England won't be able to defend all its supply centers.
Excepting Brest, all non-home SCs available to England in 1901 border the North Sea as well, unless France or Germany is willing to convoy England into some other nearby SC - a very rare occurrence indeed.
Later in the game, the North Sea remains an important asset for England because a fleet posted here can defend two home SCs as well as Norway, and at the same time force Germany to leave a unit in Belgium, Holland or Denmark for defense (assuming England hasn't taken them herself).