When teachers understand the key purposes for having students do mathematical writing, they can use that writing to deepen understanding.
Casa, T. M., Evans, K., Firmender, J. M., & Colonnese, M. W. (2017). Why should students write in math class? Educational Leadership, 74(5). Retrieved from http://www.ascd.org/publications/educational-leadership/feb17/vol74/num05/Why-Should-Students-Write-in-Math-Class¢.aspx.