Fiber helps keep everything moving and healthy inside
If you are not getting enough fiber in your diet, then chances are you that you are not eating enough healthy foods. Foods that are high in fiber are also loaded with other things you need. Beans, whole grains, strawberries, blackberries, apples - there's an endless list of ways to increase fiber through diet.
That said, if you still aren't getting your 25 grams, then fiber supplements are fine. I'd avoid anything gimmicky or expensive and stick to something like Metamucil, which also comes in pills now.