Closely tied to sea lines of communication are "chokepoints" - and there is no better illustration of these vital places on the planet than this article from the U.S. Energy Information Agency in its look at trade route "tight places" - with an emphasis on oil - at World Oil Transit Chokepoints:
World oil transit chokepoints are a critical part of global energy security. About half of the world s oil production moves on maritime routes.For more fun reading about chokepoints, see Chokepoints: Maritime Economic Concerns in Southeast Asia (National Defense University (pdf)) which is about 15 years old (but then it's not like the chokepoints move around much, it it?), from which come another couple of illustrations of chokepoints and the flow of commerce.
Chokepoints are narrow channels along widely used global sea routes, some so narrow that restrictions are placed on the size of vessel that can navigate through them. They are a critical part of global energy security due to the high volume of oil traded through their narrow straits.
Blue arrows point to chokepoints
In 2009, total world oil production amounted to approximately 84 million barrels per day (bbl/d), and about one-half was moved by tankers on fixed maritime routes. With the onset of the global economic downturn that began in mid-2008, world oil demand declined, and along with it the volumes of oil shipped to markets, both via pipelines and along maritime routes such as these chokepoints. By volume of oil transit, the Strait of Hormuz leading out of the Persian Gulf and the Strait of Malacca linking the Indian and Pacific Oceans are two of the world s most strategic chokepoints.
Now, suppose you wanted to cut off the flow of oil to Japan - where would you place submarines or ships or aircraft to impose that embargo with the least expenditure of equipment and personnel? Could it be done solely by shore bases? By aircraft?
How about Somali pirates - what key SLOCs and chokepoints can they interfere with? See Where the Somali Pirates Operate and Why, where I put this up:
There are alternatives to the Red Sea/Suez Canal route - but they are longer. See below for a graphic illustration of that point which I
Should the Southeast Asia chokepoints become closed, there are alternatives for Japan, too, like sending ships on routes to take them across the sometimes ice free Arctic route (see here) or sending ships down below Australia and out into the Pacific. Money and time, though, are both eaten up in using alternate routes.
Better to keep chokepoints and SLOCs open.
That's why we have a Navy.