You can make up a drug, and go into business of selling it, and if the general population doesn't understand logic or scientific method you can make it sound like you've got the perfect cure. If you give it to a guy and he gets better, you can take credit for it and tell the world it works. If the guy doesn't get better, you can tell everyone he just didn't take your drug soon enough, or he didn't take enough of it.
Science tells us that you should actually compare the drug to another one, or to a placebo, to see what it's effects are, but not everyone gets that.
The reason I think it's so important is because I see the quackery EVERYWHERE... like when my $1000 mutual fund only made $60 over eight years during a GOOD stock market period, people were still telling me that mutual funds and investments were still the answer, the problem is that I just didn't put enough money in, or didn't keep it in long enough. Just give it a few more years. Give it decades! The truth is, I could have had my money in any plain old savings account, taken less risk and made the same interest rate.
I see it in religious fundamentalism. I read the No Longer Quivering blog about women who tried to really submit to their husbands like they were told the Bible commands... it was awful for their marriages. It turned the guys into assholes. The whole time their religious leaders were saying that the problem was not enough submission and humility, not enough silence... the men would certainly come around and be good husbands because this WAS the cure for a marriage, they were sure of it. But things spiraled downward.
So it's not just in medicine... the quacks are everywhere. Once you know what red flags to look for they're obvious, it's getting there that's the problem.