Soluble fibers (pectin)
Do not get digested by the human enzymes, but at the same time its is a "food" for beneficial microflora in GI trackt.
Microorganisms in the intestines are averagely 3% from the human weight so that is why vegetable fibers are so important for human health.
Insoluble fibers (cellulose)
Do not digest by the human enzymes or microbiome, that's why they are called ballast.
Helps to stimulate the intestinal motility.