What this actually improves
If you have no allergy and no pest problem, an encasement will not change how you sleep, and anyone telling you otherwise is selling.
The gains are real for two groups. People reacting to dust mite allergen, where a barrier with a 4.74 micron mean flow pore stops the allergen reaching them at the source. And anyone who has stopped sleeping properly because they are worried about bed bugs, where the value is knowing the bed is sealed rather than hoping it is.
For everyone else, the honest benefit is a mattress that lasts longer and keeps its warranty.