|Such a situation would be unlikely. It would presumably require a peer or near-peer conflict (which would be unlikely mostly but not only because they would probably have nuclear weapons). Any conflict against a lesser power (or in particular against some militia, insurrection, or terrorist group) would either be against someone without ships or with ships that would probably never threaten the USMC mission because they would be destroyed before coming close.|
Potentially if you had some small USMC deployment near the coast or on an island off the coast of South Korea it could be used against North Korea's navy, but North Korea's navy (at least if they try to use it actively rather than hide it) would not likely be long for this world in any 2nd Korean War. You could also get scenarios against the Iranian navy but the same point holds for them as well.
Even among neer-peer powers Russia is not likely to present the type of threat that this would deal with, their conventional threat would mostly be deployed across land. There are some possible scenarios for small scale use against Russia in maybe the Baltics or potentially Norway, but the Russian surface fleet would probably stay on defense in any potential war.
So at the moment its pretty targeted to China. If the US tried to stop some Chinese attack against Taiwan, or the Japanese islands that the PRC claims, or you had some other island based conflict in the East China Sea, or South China Sea, or potentially further from China than that (but still in the Pacific) if the naval balance between the US and China shifts significantly, and China becomes more aggressive further away from their shores.
I don't think the capability would be worth developing if it was extremely expensive but this isn't an extremely expensive capability for military equipment. The launchers are just variants of vehicles that the USMC is going to have anyway, and the missiles are already developed.
I think having the capability makes sense for the US. It makes even more sense for other countries that are close to potential threats like Japan, Taiwan, South Korea, Finland, Sweden, Vietnam, Israel, Iran (who would want to use it against the US or against countries that we are allied to). And a number of them already have mobile ground based anti-ship missiles launchers.