>How would you continuously crush hydrogen with that?
Why do you the crush to be continuous? I thought the idea of metallic hydrogen was that it is stable once you get it in a metallic state.
there are two main hypothetical uses for it: a solid rocket fuel (with a x4 increase in specific impulse) and a superconducting wire with zero transmission loss across long distances (making solar farms in remote places attractive). there may also be military applications eg. hypersonic thermobaric weapon delivery systems.
the other replies in this thread are more realistic. the experiment is likely to study what happens to atoms in high magnetic fields, electrons for example start to pool together in strange ways. as with most scientific research, useful applications are often beyond the horizon and emerge from serendipitous discovery.