The whole point of advertising is to encourage us to buy things we don't need. Advertisers are in the business of making money of themselves and their clients. If they manage to sell us something that's good for us too, that's fine. But their primary goal is profit.
It begins when we're children. Saturday morning children's television is full of commercials. Over and over again, children see images of toys and games. They see other children who are having the time of their lives. They watch these children and get the message that if they want to have that much fun, they need to buy those things. Thus, at a very early age, we're introduced to two ideas: that we want to be like everyone else, and we want to have what everyone else has.
This continues throughout our lives. We spend a lot of our money trying to keep up with our neighbors. We buy the latest model cars, have all the latest gadgets in our homes, and live a lifestyle beyond our budgets. Advertising encourages us to define ourselves by what we own rather than by who we are. It encourages a competition of false values and shallow measurements of what matters in our lives.
Advertising can be damaging. However, it is also one of the ways in which our economy keeps growing. People need to buy products in order for other people to have jobs manufacturing, transporting and selling those products. Advertising also keeps us informed about new products that may actually help us in some way. For these reasons, you could say advertising is a necessary evil.