So in addition to any immediate practical applications there's also this element of cracking a famous long unsolved problem. It'd be like if we discovered definitive proof that P != NP, or a theoretical basis for FTL communications. Even with no immediate practical applications it'd still be huge news.
I wouldn't say that it was necessarily "theoretically possible," for there has never been, and there still isn't, a grand theory of how any given material's atomic/crystalline structure relates to superconductivity. In other words, with no theory of material superconductivity, it was never quite clear what's possible and what isn't. With this new material, though, we might get a lot closer to a working model, if nothing else.
Which would enable us to put a lot of solar power in the desert and move it around effectively to other places on Earth.
Again, with the right material properties it could also produce efficient storage, and that also has huge implications for electrical generation.
If you lose 20% to the grid, then build a 20% bigger solar farm.
Solar farms actually help with reversing desertification by reducing water losses from direct exposure to sunlight.
Maglevs are also a popular guess - safer, faster, and more power efficient ground mass transport would be a huge thing.
Maybe even magnetic rail space launches.
And, of course, military applications (the last few examples I mentioned involve acceleration of big-ish masses to surreally high velocities, which is popular approach to weaponry).