Advertisement helps in letting the customers know about your product. If a product exists in the market but the consumers have no knowledge of its existence then they cannot buy it. That is the first and foremost objective that advertisement achieves. That is, it gives out information and spreads knowledge about the product.
Advertisement can enhance the image of the product and if carried out tastefully people will be attracted to your product. A good ad will also help your product stand out among competitor products. It will highlight the specific benefits that your product offers as well as what is known as the 'competitive advantage' in marketing jargon. Competitive advantage is the quality in your product that other brands do not have to offer. It is only through advertisement that consumers can find out about it.
However, as I already said, a good advertising campaign cannot save a bad product, no matter how much you spend on advertising if your product is not good enough, it is bound to fail. Good advertising may prompt consumers to try your product but re-purchase will not follow because of the lack of quality of the product.
Therefore, the first and foremost thing in successful selling of a product is not advertisement but a good product itself, though advertisement can help tremendously in increasing sales.