A type II superconductor is known for how it can undergo quantum locking when below a certain temperature, but not much attention is given to type I superconductors. With some research, I found that type I superconductors do not have flaws like the type II variety, and that no electromagnetic fields can penetrate them. What does that mean in both a practical sense (are there any important uses of type I superconductors), and a conceptual sense (how do they interact with magnets if at all)?
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Practical sense
In type I superconductors, below a certain critical temperature and below a certain critical magnetic field, no electromagnetic field can penetrate the sample. This is a consequence of the presence of resistanceless current flow in the superconductor. So the sample can support current flow with no resistance and no dissipation through heating due to Joule effect. Such objects can be used in technological applications to support huge currents. The same holds for type II superconductors, with the only difference that they have an intermediate phase for intermediate magnetic fields, where the electromagnetic field can penetrate the sample in small regions, and this leads to a non vanishing - although small - dissipation. Nevertheless in both cases one has to cool down a sample and keep it below its critical temperature, and clearly, the higher the critical temperature, the cheaper the procedure. It turns out that most of the high-critical-temperature superconductors are type II, so they are cheaper to use, despite the presence of flaws and the intermediate phase.
Conceptual sense
In my opinion, superconductivity is intriguing for two theoretical reasons. First of all, the expulsion of electromagnetic field lines can be explained in terms of spontaneous symmetry breaking and the Higgs mechanism. A superconductor is a neat and intuitive example of such theory, which you can surprisingly find in other more complicated subjects, such as electroweak theory. Secondly, a superconductor is a macroscopic manifestation of a quantum many body coherent state: electrons in the superconductor don't behave as a bunch of independent particles, but rather as a unique thing, characterized by a coherent and collective behavior. The dissipationless current is a direct manifestation of this important property.
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$\begingroup$ Nice answer (+1) In type I superconductors, at a specific magnetic field strength $B_{crit}$, the locking effect suddenly breaks down. How do you think this can happen so suddenly at a very specific $B_{crit}$ strength? $\endgroup$– JamesCommented Sep 3 at 20:05
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1$\begingroup$ In type I superconductors there is no "locking effect" right? I mean there's no flux pinning, but the magnetic field is fully expelled from the bulk. This happens unless the magnetic field is too strong (above $B_{crit}$), in which case the field penetrates the bulk as in normal metals $\endgroup$– MatteoCommented Sep 4 at 8:25