Liner paper is highly recommended when installing on rough walls and paneling. Wallpaper has a tendency to move when the weather/humidity changes. When that happens, popping seams and possible peeling can happen. Installing a paper liner is the best way to guarantee that the seams will not lift.
Like a pad under a carpet or liner on your drapes, its unseen. It’s a best practice with certain wallpapers like grass cloth and rough surfaces but it can be optional. With grass cloth, 50% of my clients use liner paper and 50% don’t. The percentage goes up to 90% when installing on rough walls like stucco or paneling.
If you use liner paper, grass cloth will last for 30 years. If you don’t use liner paper, it all depends on whether or not the homeowner keeps their home humidity low, i.e., uses A/C all summer and humidifiers. So it’s possible there would never be a problem. That’s why 50% of people take their chances with no liner.
Wallpapering over rough surfaces or paneling is another story. If you want to be sure you won’t be fixing seams or corners every few months, it is best to use liner paper.
