One of the reasons to start OT is handwriting because it accounts for the majority of time at school. The causes for handwriting difficulty are various.
A (a preschool), who was a bright and friendly girl, still held a pencil with palmar supinate grasp (which is appropriate for 12-15 months). This caused her difficulty to form appropriate letters. She had good sitting posture (muscle tones) and functional skills for fine motor skills except one, her intrinsic muscles of the hand were underdeveloped. The intrinsic muscles are laid between the fingers and those are for dexterity. After three months of therapy at her home to improve her intrinsic muscles through activities, she was able to use an age appropriate grasp and enjoyed writing.
B was an eight-year-old boy who had received OT at school before they moved to Japan. He had difficulty with handwriting; alignment, space and size of letters were disorganized. In addition, his way of holding a pencil was not uniform. The initial evaluation revealed that he had difficulties with processing sensory information of tactile (for holding a pencil properly), proprioception (controlling and grading force) and vestibular (this affects sitting posture), along with low muscle tone (keep appropriate sitting posture). At a sensory gym, he received sensory integration therapy. Not only his handwriting and sitting posture but also his body movement was improved such as bicycling and basketball.
Both cases, children didn't practice writing to improve their handwriting, instead they worked in underdeveloped skills. Because fundamental skills are necessary for our movements and tool use, a child usually becomes capable of other things.