Wallpaper primer prevents wallpaper paste from softening latex paint. It makes the paper easier to remove and creates a sound, hard surface for adherence to drywall. It also prevents stains from bleeding through the wallpaper.
Before 1960, most walls were made of plaster. Plaster does not require priming. Sizing was traditionally applied to plaster to make the wall more moisture resistant. The sizing was composed of a thinned-down wheat paste made from flour.
After 1960, most homes had walls made of drywall. This paper-faced board required more than a coat of flour paste to protect it from the moisture contained in wallpaper. It was during this time that primers were developed to seal the surface.