I would presume we need them because every healthy human being has them after one to two years of age. We also know that antibiotics that kill gut flora lead t

I would presume we need them because every healthy human being has them after one to two years of age. We also know that antibiotics that kill gut flora lead to health/digestion issues until the gut flora is replenished. It doesn't take nearly as many why questions to get to a "we don't know" answer with biology versus physics. There's lots of stuff about biology we can observe but do not understand the why as well as we understand something like the physics behind a microprocessor. There are even theories that gut bacteria can drive human behavior at some level. https://www.theatlantic.com/health/archive/2015/06/gut-bacteria-on-the-brain/395918/ https://www.theatlantic.com/health/archive/2015/06/gut-bacte...