Fibres are very versatile as you have read in the earlier part of these small articles on them. Fibres have nutritional advantages and with that they have an important role in our metabolism. Apart from the effects on our colon and stool, the water binding capacity gives opportunity to retain water in the baked product.
The differences in soluble and insoluble fibres as well as their interaction towards viscosity allow them as well to act as a bulking agent in order to reduce fat content and with that calories. Depending on the nature of the fibre, the treatment of the fibre the can also add other functionalities then water retention, water binding, calorie reduction and metabolism. Fibres can have an influence on the volume (both increase as well as decrease in volume), but they also might enhance or even block certain flavours. With that you might be able to enhance perceived sweetness while there is little sugar present for example.
Another direction is the easy breaking of products or the phenomenon known as checking in biscuits and crackers, although certain long insoluble fibres (hemicellulases and in particular arabinoxylan) are related to this, the addition of soluble fibres can also add more internal binding and reduce the chance of breaking. The addition of insoluble fibres in combination with enzymes are able to facilitate similar results as they are being solubilised then and with that they act as hydrocolloid.
As a last big benefit is the fact that these natural ingredients do not come with an E-number, allowing you to formulate more cleaner label products.