Structure changes the eating experience
A low-fibre meal can be full of calories and still feel incomplete. The mouth gets taste and the body gets energy, but the gut receives very little structure.
Fibre helps food feel more complete because it adds texture, holds water, increases volume, slows eating, and asks for chewing. That is why whole fruit feels different from juice, dal and vegetables feel different from plain refined starch, and oats or legumes often stay with you longer than soft fast foods. Fullness is not only about calories. It is also about texture, viscosity, stretch, water, chewing, and time. Fibre gives meals more of that physical information.