Are you a biologist? A medical doctor? How would you know what to study? God, I'm so sick of this armchair science.
Here's a thought: a menstrual cycle is hormonal. Trans men have a different hormonal cycle than women, because of the hormones they are administered. So studying menstruation in XX who take male hormones helps us understand menstruation in general. I don't know if this is what they are studying, but it's certainly a plausible and good scientific reason for doing so.
So many great discoveries were found by people doing stuff others thought stupid. "Why are you trying to determine if light bends around the sun? Who cares? Surely we're not studying this." Good thing we didn't have you making decisions about what to study, because we wouldn't have relativity.
"Why are you studying the interactions between molds and bacteria? How stupid," said people pre-penicillin.
"Surely we're not spending money studying gila monster spit," said people before Wegovy.
"Why are you studying whether or not a single magnet moving back and forth on an infinite magnetic tape can compute algorithms," said people before computers.
If you're not a biologist, this should be your opinion about the propriety of biological research. It happens to be the same as mine. "Hmm, interesting. I wonder what they might get out of that study."