Because grammar instruction (as all grammar-level instruction) isn’t an end in itself; it is a means of developing a facility and base knowledge to be built upon in greater complexity and depth.
Students need to begin by learning to write in complete sentences not because a sentence fragment is a mortal sin worthy of grammar damnation. They need to begin in this manner to develop a facility, the very ability to “feel” and “hear” a logical structure so that they can develop the facility to intelligently know and feel when a fragment is appropriate to use.
In my own instruction, this means at times pointing out a fragment to a student in a draft and telling him not to change it, instead explaining why it is appropriate and encouraging him to look for other ways to use similar constructions. He knows what a sentence is and what a fragment is; he is where he needs to begin to learn where each is appropriate, and yes, to intelligently have a rightly informed sense of not only when but why he would make a choice.
To teach a student how to thus think about language is to give him access to levels of interpretation of what he reads and expression in what he writes that no merely “heard” and “felt” understanding of grammar can attain. As I often tell my students, any of the authors we read could, if asked, identify rightly why they used a construction, a phrase, a word. They may not have considered this in premeditation, but their sense of “hearing” and “feeling” was not disconnected from a knowledge of the patterns and structures, their proper and right use, and when and how to use them.
You see, our goal is not the flat, stayed, hackneyed one of seeing those “great” writers (though never denying excellence) as somehow beyond our understanding or attainment. Instead, it is to teach students to engage with others in deep and profound ways, to study not only their ideas but their craft with words and expressions, to learn from other humans how to be more human themselves and in relation to others.
We may not produce the next Wharton or Lewis or Eliot, but we hope to produce students who can read, understand, appreciate, and even seek to emulate them.