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.
If you want the best wallpaper installation possible, you want a liner.