Perfectly reasonable question. There are various ways to create book-cloth. One of them involves saturating the cloth with the adhesive and then applying the liner. This way the cloth is stiffer and may impart other desirable qualities like being able to stamp with hot foil better. However, soaking some cloths would ruin them. Another way to line cloth is by having the adhesive applied only to the liner of the cloth. This is a much drier method and works better for PVA glues rather than paste. The goal here is to keep the adhesive from soaking through the cloth so that the cloth stays pristine on the showing side. Now to the point.
Allowing this liner adhesive to dry means that there is a barrier to the next application of adhesive. It is protecting the cloth. If the next application of adhesive for the purposed of creating a cover for the book is too wet, it may soak through and damage the cloth.