An obvious reason for advertising is simply informing people of the existence of products they might be interested in buying. No one will buy something that they don't know exists, no doubt about that. When more people know about a product, more of it will be sold.
I remember an ad from an advertising agency that was about advertising itself. They said that it was good that there is advertising, as it is an important source of information about products. If that were all, most people would embrace advertising as a welcome addition to their lives.
However, advertising is obviously not a source of objective information. The bright side of a company's products is highlighted exclusively. No ad will list both the pros and cons of a product. It is also not exactly a source of complete information, as the amount of actual information in ads is usually very minimal, if, in fact, there is any real information at all. So, ads are not particularly trustworthy as information.
But advertising goes much further than being a limited form of information.