Good products hardly ever take out on their own. I'm not sure what computer you're logged on from, but to take that as an example, I'm pretty sure it's made by a company that would likely go under if it chose not to advertise.

Advertisement is sometimes just to inform and build awareness, sometimes to build brand, but often to persuade, and sometimes it exploits unethical persuasion techniques. The gap between ethics allowed by society and your own personal ethics are very variable from place to place and person to person. For example in a lot of places it's illegal for doctors and pharma to advertise; some might consider it a good thing not to exploit hypochondriacs, but is it ethical if it results in patients not knowing about an actual cure for their condition? These things are endlessly debatable, they're very far from a black and white situation.