Fiber helps with weight control mainly by making you feel fuller. This, in turn, makes you less hungry and therefore less likely to eat more.
Fiber-rich foods such as fruit, vegetables, potatoes, wholegrain breads and brown rice contain few calories for volume. Because they are bulky, they keep you feeling fuller for longer. You’re also inclined to eat less of high-fiber foods because they are so filling. Even the extra chewing time can contribute to feelings of satiety (fullness).
It's also true that foods low in fiber often do the opposite of fill you up. Although generally higher in calories, low-fiber foods such as candy and chips are less filling and so you're likely to eat more of them.
You can see a good example of this when you compare an apple with a glass of apple juice. Two or three apples are needed to produce one glass of apple juice and in the juicing process, all the fiber is removed. (The sugar and calories become more concentrated.) When you choose the apple juice instead of the apple, you consume more calories but are less full.