Recently I had a bad cavity (back of mouth, didn't hurt until gigantic, don't go to dentist regularly) and very minor bone loss in my jaw next to it. In the process, a dental hygienist explained that the primary purpose of flossing or waterpicking is to prevent bone loss in the jaw which can lead to infection and tooth loss if left untreated.

Thanks to waterpicking, the gums in that area of the jaw have healed and I shouldn't experience any more bone loss as long as I waterpick or floss.