It's more important than I realized: In addition to providing bulk to promote regularity and satiety or fullness, it's involved in the removal of excess hormones, carcinogenic substances and vitamins (excessive amounts).
In the past, every time I read about B-12 the information was that we can't manufacture enough in our own bodies. But last night I read something different from the book: "Stopping Cancer Before it Starts":
"In some cases, fiber actually creates nutrients we need. Vitamin B-12 is an example. Gut flora feeding on and fermenting or otherwise transforming particular kinds of fiber produces most of the B-12 we need."
The above quote is probably true, assuming that you maximize the amount of fiber in your diet. Unfortunately, most people don't.