Bunkers are a prime example. There's a reason why light cruisers water the gold standard in fire support for amphibious landings - the 6 inch shell is a very useful size. You can also do interesting things in a 155mm shell that aren't quite practical in 127mm, like terminal guidance, dispenser rounds, or tactical nuclear weapons.
Big guns were the standard for hard target destruction, but by the 70s other options were available such as anti-tank guided missiles and laser/EO guided bombs. This pushes NGFS back to the role of regular artillery support, engaging targets in the open or providing suppressive fire. In any case the need to engage hard targets is not a prerequisite for every amphibious assault, the landing can take place in an area where there are no really hard targets and the troops can move to the objective.