What is the purpose of publishing in such journals?
There appears to be a mismatch between your view of &quot;Math&quot; and what math means to a broader community. Sure, there are those doing leading edge research, publishing bold new results, and so forth. But, there are also those who have a calling to do things like teach mathematics, including dedicated high school teachers. Why should there not be a journal for them to read? And if there is such a journal, somebody has to write for it, and those people are normally professionals closely engaged with teaching (remember that many professors, including in math, are at teaching-focused institutions).
In a non-math context, I would point to the American Association of Physics Teachers (aapt.org), associated with the American Physical Society. Their mission statement reads (AAPT Mission):

Mission:
AAPT's mission is to enhance the understanding and appreciation of physics through teaching.
When the organization was established in 1930, our goal was clear: &quot;ensuring the dissemination of the knowledge of physics, particularly by way of teaching.&quot; Decades later, we remain committed to that core value, but with a new emphasis and meaning provided by our current mission statement.
Our vision is to be the leader in physics education. We are committed to providing the most current resources and up-to-date research needed to enhance a physics educator's professional development. The results are not only a deeper appreciation of the teaching profession, but most importantly, more enthusiastic involvement from their students.
The Association has identified four critical issues that will guide our future activities:
Increase AAPT's outreach to and impact on physics teachers
Increase the diversity and numbers of physics teachers and students
Improve the pedagogical skills and physics knowledge of teachers at all levels
Increase our understanding of physics learning and of ways to improve teaching effectiveness
The twenty-first century will provide the greatest opportunities and challenges for us as we take an active role in shaping the future. Our success will depend on the commitment, dedication, and continued input of our members and the physics education community. Join us in this journey to enhance the quality and effectiveness of physics education at all levels.

They publish several journals, the American Journal of Physics and The Physics Teacher, focused on slightly different audiences within their society. Frankly, some of their stuff is pretty interesting (in the spring there was a nice paper on the production of polonium in the Manhattan Project, physics included).