Many wallpaper instructions mention the use of liners, also known as blankstock. The recommendations vary from the generic, "For best results, we recommend lining the wall", to the definitive, "walls must be lined".
Why SHOULD a liner be installed under the finish paper? The simple answer would be, "because the manufacturer recommends it and without liner, the manufacturer is not obligated to consider any claims against unsatisfactory performance or appearance of the wallpaper."
But that still does not answer the questions, "Why a liner? What does it do? What is its function?"
A liner has positive effects on the aesthetics and longevity of the installation.
It is often erroneously explained that a liner will smooth out rough walls, bridge grooves of wood paneling, or hide textured surfaces. Although a heavy-duty non-woven bridging liner may help alleviate those situations, the subject at hand is blankstock liner, which as the name implies, is blank, unprinted, stock pulp paper. It is suitable for smoothing out the pitted appearance of some plaster walls and will soften the hard edges of chipped paint that were never feather-sanded. However, no liner will smooth out bumps, grit, nubbins, and other protrusions.
Liner will give a uniform surface appearance to the finish paper. It has been called, "a soft upholstered look".
Perhaps the most beneficial aspect to liner is that it fights seam-splitting of the finish paper. Like all paper, wallpaper, when wet, expands. When it dries, it shrinks. This natural process wants to pull the seams apart. The goal is to securely anchor the paper before it has a chance to shrink. Because blankstock liner is absorbent, it quickly wicks moisture away from the pasted finish paper. This locks down the paper in its expanded state, preventing shrinkage and seam creep.
This quick removal of moisture from the finish paper also lessens the chance of water and paste migrating to the surface, which can cause blushing, mottling, or other staining problems on some sensitive materials.
Many hangers (myself included) have had a few experiences in which some pulp papers installed in humid environments (bathrooms, ocean side homes, etc) without a liner have stained within five years. No one has yet indisputably pinpointed the cause of this phenomenon, but the use of a liner has proved to virtually eliminate the problem.
Liner paper can also stabilize unsound surfaces. As mentioned above, when paper dries, it shrinks. Wallpaper will never fully shrink back to its original size once it is on the wall and the paste is dry, but it wants to. This puts quite a bit of lateral force on the wall, and this force does not go away with time. It forever pulls. Any paint under the paper that is weakly adhered to the substrate will be pulled off the wall and the wallpaper will fail as a result. When a liner is installed under the finish paper, the two layers of paper will actually pull against each other and negate the net forces on the wall. This is a tough concept to understand at first read, so I point you to my friend Jim Parodi's in-depth explanation: Better Living Through Blankstock
Simply put, when wallpaper and liner are used together, they become one piece of paper, similar to a sheet of plywood.
Considering manufacturers' recommendations, the look a liner gives, and the problems it solves, it is easy to see why many experienced installers and manufacturers say, "if you want the best wallpaper installation possible, you want a liner."