Fibre can be divided into
Insoluble fibe (cellulose and hemicellulose)
Soluble fibre (gums and pectins)
Insoluble fibre helps the intestines to function and helps you to avoid constipation.
Soluble fibre helps to reduce the absorption of nutrients (especially lipids). This can reduce the risk of atherosclerosis. So, if you eat a carbohydrate with a high Glycæmic index you could, in effect, lower the index by also eating fibre. The fibre you eat at the same time will lower the amount of insulin in the blood.
The foods that are rich in fibre are also rich in vitamins, mineral salts and micronutrients. If you eat fibre you are also avoiding other serious deficiencies.