why are some bacteria considered to be "friendly" to our digestive system?

Question

is it because they aid in the digestive process by helping digest lactose (milk sugar) and protein and helps to clean the intestinal tract, to purify the colon, and to promote regular bowel movements

Answer 1

http://en.wikipedia.org/wiki/Gut_flora
read the introductory paragraphs.
Beneficial is a much better word than friendly.