Advertising will increase sales but let me explain through a simple analogy.
In general advertisement increases the probability of your business to makes sales through increasing the amount of people who know of your product.
If you have 10 people and 10 people do not know about your product you will not increase your sales.
However, if 10 people see your ad there is now 10 people aware of your product, now you will have a chance to make 10 sales.
If 1/10 people need your product now your advertisement just increase your sales. If 9/10 buy later your advertisement has just completed 9 sales.
As marketers are goal is to know how people buy, and get them to buy through advertisements.