Rosary booklets are not necessary for praying the Rosary, but they are a helpful aid as they provide explanatory and even meditation points on the Rosary and Rosary Mysteries. They have a twofold purpose: they are educational, and they help keep the focus on what is being prayed.
Firstly, Rosary booklets teach beginners how to pray the Rosary; they address which prayers to say on which rosary beads, and so forth.
Secondly, booklets often enlighten the reader about the Mysteries prayed through basic and thematic elements. In these cases, booklets are great at guiding the meditation. They might have pictures that help the imagination to hold the Rosary Mystery in prayerful contemplation throughout the entirety of the decade. If the mind wanders, a simple glance at the picture is enough to refocus one’s attention. The booklets may even have Scripture passages, or quotes from saints to deepen the understanding of what is going on.