Innovative teachers can make writing an invaluable part of math instruction.
One reason I chose mathematics for my undergraduate major was that it didn't require papers. Math homework called for solving problems or proving theorems, and that was just fine with me. Math and writing, like oil and water, seemed to have little in common. And for my first 20 years as a middle school and elementary school teacher, writing played no role in my math teaching.
Today, my view has changed completely. I can no longer imagine teaching math without making writing an integral aspect of students' learning.