Storing your filament in vacuum bags works great but adding desiccant to that bag is even better. The Spool Shield puts desiccant right up against the filament instead of adding it to the inside of the core. Inside the core the desiccant is completely sealed off from the filament! Having it right next to the filament on the outside allows for actual humidity management.

Print as many as you want. If you live in a humid environment then adding more than 1 Spool Shield is recommended. Some filaments are more hygroscopic than others, for example TPU will absorb water much faster than PLA.