$\sf MM^{++}$ is a 'good' set-theoretic axiom because it is 'Maximum'. Of course, bigger is better. But I'd like to know exactly how the argument works.
Let me be clear about the question posed:
- What intrinsic and extrinsic justifications do we have to support the claim that $\sf MM^{++}$ is a necessary set-theoretic axiom?
How does $\sf MM^{++}$ clarify the definition of set, what new mathematical results does it bring, and what absoluteness does it bring?
For those who do not understand the history of Martin's Maximum and the differences between the variants, please read the first 8 pages of https://arxiv.org/abs/1906.10213.