You shouldnt be downvoted for asking an honest question. I'd answer this by rephrasing it as "What's so bad about online advertising? All they're doing is showing me targeted products that are likely to interest me." Well now think of spyware, intrusive ads with sound, popups, etc.

The general answer is that medicine is supposed to help you. But advertisers aren't paid to help you. Theyre paid to get you to buy things. And all the trickery and marketing tactics aren't conducive to getting you the correct treatment.