Question

The human gastrointestinal tract is full of different species of beneficial bacteria. How do these bacteria help in digestion?

Answer 1

in fact they do not help OUR digestion. they are benefical in other ways.
1) they feed on substances we're not able to degrade and to assimilate. they can use these substances to sustain themselves and the sideproducts of their metabolic pathways are vitamins, for example. we need vitamins and we cannot produce them for ourselves. both humans and organisms share sideproducts for the common interest.
2) the gastrointestinal tract contains many different kind of organisms. their relative quantities respect a delicate balance. if this balance is not "healthy" people can become ill. the balance has proven to be linked with cron disease and obesity.
3) gastrointestinal microrganisms are the guards of our intestinal epitelium against pathogens which try to pierce it. they hunt these pathogens and therefore help our immune system.
they're not interested in our digestion. they just live in a habitat which is perfect for their needs. protection, food, heat. in turn they protect their habitat from invaders and the wellness of our guts