We used this in Gitlab CI. Unfortunately, the only way they deal with artifacts is by putting them in Zip files. Cache between builds would thus be stored as a Zip file. However, fully extracting it before each build would sometimes take as much, if not more time than to just build fresh. Mounting a Zip file as a filesystem allows extracting entries on-demand, at the time a file access would've been made. This was a notable speedup in our compilation process.
It reduces the code required to convert from N-producers to M-consumer from N x M to N+M because they're reading from and to a well-understood common form.
As opposed to extract contents and then work on that (requiring extra steps + disk space). Or be limited to what specialized utilities support.