They are deeply unnecessary, and a little embarrassing. There are divider pages denoting every jump between the 1958 and the 1985 timelines, and each has a melodramatic epigraph from William Carlos Williams, Virgil, a classic rock song, or the 1973 Scorsese film Mean Streets. It took 350 pages for the seven main characters (too many!) to individually meet the central monster and then collectively acknowledge its existence, and we frequently took extended breaks to talk about architecture.
There is no way an editor even glanced at this book before it was published. Now is probably a good time to point out that Stephen King is out of control.