Peppermint tea relaxes the muscles of the digestive tract, which stops cramping and IBS so that food can pass smoothly and easily, which gives it time to be fully digested.
It also increases bile flow! so that foods get more digested.
It reduces the size of gallstones, so that increases bile flow and the bile itself is cleaner than before. Contaminated bile does not do its job properly, reducing the power of digestion.
All that adds up to healthy stools. :)
Here's a good page on the benefits of peppermint tea: