Hanging wallcoverings is a lot like building a structure. It begins with a proper foundation. Without it, the structure will fail and often does. 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.