Answer :

Why do firms spend money and time to advertise have a lot reasons. But a simple few can be:
1.  
Knowledge. Advertisement can bring information of a certain or set of products a firm or company is selling.

2.  
Priming. Advertisements also have a psychological effect on its customers. Priming is one psychological construct that influences ones unconscious behavior.
3.   Salesman in Print. According to Kennedy, advertising is like a salesman in print because it introduces and entices one customer to avail the product.