It actually makes alot of sense. Just as coral colonies wage perpetual war against their neighbor species for space to proliferate, so do bacteria colonies. The constant surge and ebb of both BB and pathogenic bacteria determine the overall health of the system. Basically, fish get sick when pathogenic bacteria exist in greater population numbers than BB. And, fish are healthy when BB colonies outnumber pathogenic bacteria. The regular use of probiotic additives ensures that the BB has the greatest chance of continually out-competing the pathogenic bacteria.