It blocks the physical manifestations of stress - increased heartrate, sweating, etc that can form a feedback loop, and downregulates a bit of thyroid hormone production. How does one know they're stressed? The feeling of those symptoms.
I imagine it could help, and probably be better than therapy alone, but lessoning of the negative feelings around painful memories can be treated with opiates too (which are far more addictive than MDMA, despite MDMA being far more euphoric). The problem is PTSD, anxiety disorders and personality disorders are incredibly intractable with the tools we have - if a therapeutic session could have 10x efficacy and safely get someone to a healthier state in 1 year rather than 10, it should absolutely be looked into.