Lowers both. Roughage or fiber in your diet is supposed to be associated with a reduced risk of colon cancer. A high animal fat, low fiber diet is associated with increased risk. However, direct effect of diet is still unproven. Fiber helps to clear out the colon of matter that might remain and irritate the colon, clearing out cholesterol/fat so the body cannot absorb it, lowering it in serum/blood.
Multiple factors. Nobody knows for sure, but there are some very reasonable proposals. Roughage, or insoluber fiber, both acts as a sponge, and speeds up the transit time through the intestinal tract. Fiber also impacts the kind of bacteria within the colon. Chronic inflammation can lead to cancer; speeding up the transit time through the colon reduces this risk. The "sponge" action can help lower cholesterol.