It is important because writing is not natural, like speak and listening. It must be learned, generally someone need to teach us how to write.
The most important thing that the writing teacher needs to know is "WE ARE NOT DEALING WITH ESL BUT RATHER TSL: THINKING IN A SECOND LANGUAGE. IF WE CAN GET OUR STUDENTS TO DO THAT, WE SURELY TAUGHT THEM SOMETHING. (Raimes, 1985:92).
What is the best way to teach writing to children? to teenagers?
The best way to teach writing is leaving them to write in a free way, to leave them explain their lifes, their favorite things that they like the most, etc...
Because in that way they will demostrate that they really want to learn how to write in other language, because they have fun do it these kind of activities.