Here is the thing, we need both solubale and insoluble fiber since they serve different purposes for the body.
So first of all, fiber cannot be digested by human enzymes and ends up in the colon. So why do we need it? It promotes healthy gut function by stimulating peristalsis (muscles of the colon), it increases transit time (get it out quick), promotes healthy gut flora, reduces cholesterol & colon cancer, and helps maintain weight.
Soluble Fiber: absorbs water and helps build the feces. Found in oats, fruits, beans, seaweed, pectin.
Insoluble Fiber: does not absorb water, increases bulk of stool and speeds it up so it gets out of the body fast. Found in wheat, rye, vegetables.