I take psyllium husk in my protein shakes to thicken them up a bit. With what I eat, I'd say that I get around 40-60g on a normal day.
Fiber is a great tool for dieting. Fiber has a high thermic effect because your body cannot absorb any energy from it. Basically, you burn calories eating the stuff. It may not be much, but for someone trying to cut, it gives them another advantage.
Also, soluble fiber will slow the digestion rate of food in your stomach, which in turn will lower the rate at which sugar is released and absorbed in the blood. This means that it will decrease the Glycemic index of that meal, which is a big thing nowadays in the fat loss & bodybuilding community. Insoluble fiber helps with moving bulk through the intestines.
Fiber has other benefits too, but those are some of the more relevant ones.