It's defense through deterrence - i.e. the opportunity cost of forcefully acquiring control of the largest-GDP nation is quite high.
In a post-nuclear world, this is incredibly complex so it usually ends up playing out in different theaters (Korea, Vietnam, Iran, Syria, etc.) rather than the shores of the U.S. as you pointed out, but the total reserve and reach of that force is still just as important.
The argument can get slippery though when trying to justify allocating resources, because with deterrence it's hard to know exactly how much you need, especially when gaining that knowledge usually comes through defeat of some sort.