According to PBS Parents:

Most people never consider the complexity and difficulty of the writing process. In fact, relative to all other academic activities, writing requires more basic skills than perhaps any other.

Even during their earliest handwriting exercises, children must combine complex physical and cognitive processes to render letters precisely and fluidly. As writing tasks become more difficult, students must call on an increasingly wide range of skills to not only write legibly, logically, and in an organized way but also to invoke rules of grammar and syntax. This combination of requirements makes writing the most complex and difficult use of language.

It is probably no accident that many adults have chosen jobs that limit the amount of writing they have to do. Unfortunately, children have no such luxury. From first grade on, they write nearly every day and they are asked to do more with this skill than with any other except reading. And as children progress through school, writing requirements — from homework assignments and class work to note taking and tests — increase across the curriculum. Even newer high-stakes tests are moving towards requiring more answers in the form of short paragraphs and essays.

Like all learning problems, a writing disability can be devastating to a child’s education and self-esteem and can dramatically limit what that child can achieve later in life. School requirements demand a high level of writing proficiency, and a child who struggles with an unrecognized writing disability will find it increasingly difficult to express his knowledge on many subjects, as the writing process itself will stand firmly in the way of learning.