The first set of reasons relates to the writing skill itself. The second set relates to foreign language learning in general. We can't expect young learners to develop their writing skill without teaching them how to write and giving them opportunities to practise. Learners do not necessarily transfer their skill in writing from what they can do in their own language. In the case of young learners, they may not have developed their writing skill, even in their own language.
Writing is probably the most personalized, creative activity in the language class.
By developing their writing skills early, young learners are making a valuable investment for their future studies. Writing gives learners the opportunity to find ways of expressing their ideas in a foreign language. Writing gives learners the opportunity to try out the language with plenty of thinking time. A learner's writing gives the teacher a good opportunity to diagnose grammar and vocabulary problems and to identify progress. Writing allows learners to practise new structures in an extended context.