What is Public Health and Why?

Marion Nestle's explanation (bold added):

public health makes it easier for individuals to make healthful food choices for themselves and their families. Or to put it another way, public health makes better food choices the default.

The classic example of a public health intervention is water chlorination.  As individuals, we could all boil our own drinking water to kill harmful organisms but this requires us to have stoves, pots, and fuel, and to know how to boil water.  For many people, having to do this would be an intolerable burden and responsibility.  Instead, some societies choose to take public health measures to ensure that drinking water is safe at the tap for everyone.
Other food examples: milk Pasteurization, banning of trans fats, food labeling. ...

Preventing obesity is another example: We could, as a society, take measures to make it easier for people to eat more healthfully and be more active (public health) or leave it up to individuals to do this for themselves (personal responsibility). Many of the arguments about suggested public health measures to prevent obesity are about how best to balance society’s needs with individual rights.  But as I see it, the proposals aim to tweak societal choices that have already been made: which crops receive farm subsidies, for example.... 

Obviously, both public health and individual approaches are necessary, but the overall objective of public health is to make it much, much easier for individuals to make better health choices without having to think about them. Because public health applies to everyone, it is essentially democratic.   And that’s one of the reasons why I think it matters so much.