Actually you've been told only a very small part of the story. Inside the gut are trillions of live microorganisms that promote normal GI function, protect the body from infection such as harmful bacteria and viruses, and regulate metabolism and the immune system.
In fact, they comprise 70-80% of the immune system. Only a very small percentage of these microbes are actually harmful to the body. Without these "friendly" microbes or what's called a balanced microbiome, your gut becomes sick and in time your immune system becomes compromised and you end up sick too. Replenishing and balancing these microbes within the gut is actually vital for normal function.
Stomach disturbances like gas, bloating, constipation, diarrhea, and heartburn can all be signs of an unbalanced gut. Sugar cravings. Unintentional weight changes. Sleep disturbances or constant fatigue. Bad breath. Food allergies or sensitivities. Moodiness, Anxiety and Depression. Skin problems like eczema. Diabetes. Autoimmune disease and suppressed immunity. And even certain types of cancer.