Kefir contains a lot of benefical organisms (bacteria and yeasts), and acids resulting from fermentation of the Sugar by these organisms. Therefore is it a great help to building the flora and help them control the pathogenous yeasts and bacterias in the intestines. The acids also help to restore a proper pH in the bowel, otherwise candida for example can grow and mutate, if it it too alkaline.
It contains a very little amount of lactose due to the fermentation if you let it ferment a sufficient time (24h at least). It is even suitable for diabetics or galactose intolerants.
It has many other benefits!