Edible fruits are naturally 'designed' to be consumed, so that the eater will spread their seeds. A gift for a job to be done.
In this 'present' fiber makes up the wrapping and the structure ('the box'), and water and nutrients make up the contents. Without fiber, there is no structure nor protection against insects, worms etc. But that fiber is not part of the treat; as thats the sweet juice (water and nutrients).
Fiber is not a nutrient (except for some microfibers).
Unlike nutrients, fiber cannot be decomposed enzymatically (by digestive juices). Fiber can only be partially decomposed bacterially. Fiber mostly just ends up as waste material, just as the wrappings of a present.
The problem of the bacterial decomposition of fiber is that it originates gasses. Gasses may cause bloating. Bloating will stand in the way of a flat tummy.

Don't we need fiber to prevent constipation?
Water and oils contain no fiber, and yet drinking water or oil will never cause constipation.
If you are healthy, your bowels never stop functioning. They keep on working 24/7. Even when you don't eat for a whole day, your intestines are still functioning perfectly well.
Only if you substantially sedate your bowels, you may get constipated.
Your diet conatins various sedatives, including beta-carbolines (cooked foods) and opioid peptides (dairy products, grains). Depending on your sensitivity for sedatives and stimulants, and on your intake of sedatives and stimulants, you may get constipated.
So, it is not a lack of fiber that makes you constipated, but a too great intake of sedatives.
If you are susceptible to these sedatives causing constipation, for starters, stop consuming wheat (and other grains) and dairy products.

Doesn't fiber inhibit appetite?
Fiber fills up your stomach. True.
In as much that drinking a lot of water will fill up your stomach.
When done so right before a meal, it indeed may help you to eat less. However...
A shrinking stomach triggers appetite. The more you fill up your stomach, the more it will shrink afterwards, and trigger your appetite.
To evoke less appetite, you need to keep your stomach small, by eating small meals.

To prevent bloating, minimize the intake of fiber. For a flat tummy, juices are better than whole fruits. Veggies better than legumes. Potatoes better than rice.