Wiki says: <<Soluble fiber (fermentable fiber or prebiotic fiber) – which dissolves in water – is generally fermented in the colon into gases and physiologically active by-products, such as short-chain fatty acids produced in the colon by gut bacteria. Examples are beta-glucans (in oats, barley, and mushrooms) and raw guar gum. Psyllium – a soluble, viscous, nonfermented fiber – is a bulking fiber that retains water as it moves through the digestive system, easing defecation. Soluble fiber is generally viscous and delays gastric emptying which, in humans, can result in an extended feeling of fullness.>>

https://en.wikipedia.org/wiki/Dietary_fiber

It sounds like both soluble and insoluble fiber are important for different reasons.