Maria Montessori famously said, “What the hand does, the mind remembers”. The hand reports to the brain; the brain guides the hand; the cycle continues, resulting in the development of the intellect.
Thus, while learning to write, it is important to not jump too quickly into pen-paper work before building and refining the child’s muscle memory using their hands. The “sandpaper” letters give the child a concrete, tactile experience that helps imprint alphabet patterns in the brain.
In this case, this child is working towards mastering his understanding the letter “v” by tracing the corresponding sandpaper letter. This is critically important preparation before the child physically engages in writing the letter on paper or slates (and the coarse sandpaper is very enjoyable to run their fingers through!).