When put like that, nuclear missiles which are fired after we're dead sound useless because they would just mean even more people dying. While that is correct, a submarine deterrent could actually lower the death toll in a nuclear war. Why? Because the USA relies heavily on land based Intercontinental Ballistic Missile (ICBM) launch sites which are well known to the military planners of potential adversaries, these sites would be among the first to be hit in a nuclear war. Since American leadership knows that all their land-based nuclear missiles would be quickly destroyed by incoming strikes, Donald Trump would have no more than a few minutes to decide whether to launch those missiles or never use them at all.
A submarine deterrent could actually lower the death toll in a nuclear war
This "launch on warning" dilemma is dangerous because if Donald Trump knows he has mere minutes to decide, he (and any other decision maker) may act aggressively and fire more missiles than if there was enough time to make thoughtful & strategic decisions. It's also worth noting that contrary to films like G.I. Joe: Retaliation, once nuclear missiles have been launched, they cannot be cancelled or recalled by those who launched them, meaning rushed decisions are permanent.
If all the world's nuclear missiles were on submarines, leaders would probably take more than a few minutes before deciding whether to retaliate because, unlike nukes based on land, submarine ICBMs wouldn't be destroyed by that first strike. The more time world leaders would have to make those astronomically consequential decisions, the better. If the worse came to pass, extra time might also give leadership the opportunity to have pragmatic crisis conversations and find better solutions than launching more nukes which wouldn't bring back those already killed.
If the world has to have nuclear weapons in the first place, it would seem to make sense that all of them should be in submarines for aforementioned reasons. However, many nuclear armed nations choose to diversify their nuclear deterrent because warfare is constantly evolving and could suddenly undermine a given deterrent, leaving them vulnerable if they rely on deploying all their nuclear weapons in one way. For instance, rapid advances in remote sensing technology and military AI mean that submarines could become easier to detect and destroy in future.
The future is certainly very difficult to predict, all we can be sure of is that unexpected things will happen at unexpected times. However, not all of those unexpected things necessarily have to be bad. Who knows? Someone might come up with an energy-particle beam which disrupts the trigger mechanisms of nuclear bombs and renders them all expensive paperweights.