As far as I know, it is believed that the moon stabilizes Earth's tilt angle. Without it, the Earth will wobble uncontrollably (over long periods of time) as does Mars now, dramatically changing our familiar climatic zones.
“The moon has long been recognized as a significant stabilizer of Earth’s orbital axis.
Without it, astronomers have predicted that Earth’s tilt could vary as much as 85 degrees.
In such a scenario, the sun would swing from being directly over the equator to directly over the poles over the course of a few million years, a change which could result in dramatic climatic shifts.
Such shifts have the potential to impact the development of life.”
https://www.space.com/12464-earth-moon-unique-solar-system-universe.html
I believe that's a canard, one of those, "everything must have a purpose" kinds of just-so stories.