It has to do with improper or lack of preparation. Every paperhanging job can benefit from a wallcovering primer. True, there are some surfaces that don't require priming. The foundation may already be laid if the wall is structurally sound and has a fresh coat of flat paint with a lot of binder, for example. Some manufacturers recommend priming instead.
Sealers form hard, slick surfaces that have the potential for bond failure once the paste dries.
Wall sizes, on the other hand, simply reduce the porosity of the surface by filling holes. Primers give better adhesion and protect porous surfaces, making strippability easier. Even with their hard-working qualities, wallcovering primers can't do their job if the initial prep work is left undone. Be sure to properly clean the wall and disinfect it against fungus, mildew and bacterial growth. Sand any rough spots and fill holes and gaps.