Research in recent years by psychologists, educationalists and neuroscientists has found that older children, with better handwriting skills showed greater neural activity in areas associated with working memory (used for planning, ideas generation and composition skills for written work).
Due to the way that our working memory functions the handwriting process can impact on the quality of the work. For instance, those who have poor handwriting ability use a disproportionate amount of their working memory capacity in recalling and forming the letters, effectively blocking the higher level composition process (Gathercole, Pickering, Knight & Stegmann 2004, cited Medwell et al. 2007).
This is because children with fluent handwriting skills have developed an automotive (instant, subconscious) ability to recall and reproduce letter patterns, making handwriting a lower level process within their working memory.
This would suggest that learning to handwrite with accuracy, fluidity, speed and legibility is a vital goal if we want our children to reach their full potential. Learning to join letters is therefore an important step to achieving this. Once handwriting has been mastered a child can focus more effectively on the composition and structure of the piece, which requires planning and logical thought processes, so that the plot or argument can be fully explored and presented.
Medwell. J, Wray. D: Handwriting: what do we know and what do we need to know, Literacy Vol. 41, No 1, April 2007.