> ... you set up an "interpreter" to handle the statements in your function however you want: for reals, for test, for reals with logging, etc.

That is the tip of the iceberg of free monads. Their full power lies in being able to combine different type sof effects into more powerful, composed effects. E.g. you want to do IO while also processing probability distributions using a probability monad. But they can get pretty hairy. See https://youtu.be/qaAKRxO21fU for the gory details.