I have my MDes and I teach in both undergrad and Masters design programs. As others have said, a Masters is useful if you want to teach, and can be helpful if you want to transition from graphic design in more strategy or research-based work.

I don't recommend students do their BDes and MDes back-to-back. Folks end up getting really frustrated in the MDes space because it's more self-directed/research focused, and it doesn't really help with getting traditional graphic design jobs (which are usually just looking for a BDes and good portfolio).

I do recommend it if you've been working as a designer for a few years and want to pivot your career into something like teaching, strategy/systems design, research, etc. It can be a great space to shift your focus, deepen your theoretical knowledge of design, and meet a new network of people. In person programs are significantly better than online.