Understand the argument, but that doesn't mean they are necessary. There have been groups of people who lived as carnivores, especially in artic regions. Can't say the same about vegan populations. 

Certain types of fiber are what's called prebiotics which feed the probiotics or the good bacteria in the gut. 

ProofOfWorkFitness

The argument that you should eat fiber because your body cannot digest it (and therefore it will add bulk to you stool which is a good thing, because...) doesn't make sense from a logical standpoint. Yet this is what has been said for decades now.