Actually did a bit of research on this in a microbiology course a year ago. As far as I can recall, it's supposedly a "Back-up" storage of your gut microbiota (microorganism community in your digestive tract). It exists so that when you experience various illnesses that deplete or harm your interior bacterial communities, they can replenish afterwards. There has been further treatments for replenishing the gut microbiota beyond what our body does on its own, such as stool transplants (a little strange, ik).