Writing is not about spelling, grammar, penmanship, or vocabulary. It's about having something to say and then having the skills to communicate that idea.
Those skills of course are spelling, grammar, a rich vocabulary and hopefully legible handwriting (where applicable in this digital age). Having a reason to write, something to share, is the drive behind written expression.
Using the learner's natural need and interest to communicate, to teach the very skills needed for effective communication, is an ideal environment for context-based writing instruction. And it's how we prefer to do it.