Do I really need a pillow protector if I already use a pillowcase?

Across the reviewers we read, a zippered protector adds two things a pillowcase doesn't: a barrier against sweat, oils and saliva that would otherwise soak into the pillow itself, and an enclosure against dust mites and (with the right construction) bed bugs. r/Allergies threads in particular treat encasements as non-optional for people with documented dust-mite sensitivity.