Handwriting is a complex task – i.e. it has a number of components. Thus, children with poor motor skills will often struggle with this task. But some children struggle with handwriting because of ‘language’ difficulties. The difficulty is that both the motor component and the language component are cognitively demanding in younger children – it is only later that these skills can be described as ‘automated’. This means that a child with poor motor skills will need to concentrate on their hand movements which adversely affects their ability to form coherent sentences etc. Conversely, a child with language difficulties will need to concentrate on their sentence construction which will have a deleterious effect on their pen skills.
It can be seen that handwriting problems have a great potential to hinder a child’s ability to progress within the classroom and therefore need to be addressed.
I think your question probably relates to the motor components of handwriting. Poor handwriting is often a sign of a more general problem with motor learning. If the problem is not associated with a known condition (e.g. cerebral palsy) then this can sometimes be given a diagnostic label (e.g. developmental coordination disorder) though the majority of children do not have their problems recognised in this manner (owing to failures within the system).
In this context, handwriting problems are indicative of more general difficulties and it is known that these difficulties can have an extremely deleterious effect on a child’s wellbeing. The good news is that motor problems can be treated using pedagogical techniques (practice helps, make intervention fun, target the key skills that need to improve etc).
I hope that helps (a full answer would be many thousands of words)!