a) Shipping. Big container ships can't be sail-powered (the largest sailing ships ever are about 1% of the size we'd need). But existing marine nuclear power is a proliferation and/or meltdown risk. If we can make a small fusion reactor, we could make our existing global supply chains carbon-neutral.
b) Continuous industrial processes that can't survive a power glitch, much less nighttime on a windless night (no renewables). There are a lot of those.
