Because stagnant water is better for bugs to live.
Jokester
OK I guess I didn't word that the best since yes it's going to be better for you than stagnant water
Just be aware that boiling/purification isn't a guarantee of truly safe water, you really need a combination of them and an effective filter such as the one 6thElement linked to. Chances are you could drink from a stream repeatedly and suffer no ill-effects but the one time you do and get cryptosporidiosis...you'll wish you had been more thorough