How important are the demographic factors in my marketing campaign?

Very Important, as any marketing campaign comes with a finite budget, identification of the target market demographic is always the first step in planning a campaign. Any further activities should revolve around reaching the maximum number of the target market demographic within the specified campaign budget.

Well Demographics is the the ultimate categorization tool for any Marketing Campaign. Grouping consumers based upon characteristics allows to better serve the needs of their ideal customers. In terms of marketing, it means selling a product or service to the person most likely to buy it based on their unique needs and preferences. Better you know your customers, the more likely you can give them exactly what they want.

Sachin and Carlos have both touched the base. Good answers

The Demographic factors are very important in the marketing Campaign due to the following reasons :

1- Categories of population are known as well as preferences to meet the customers needs 

2- Focusing on the new trends of the market to create new products and services

3- Further identification for the New customers requirements to be considered for the future campaign

4- Limitation of the Target Budget as per the real needs of the market for the certain Products and Services

5- Knowing shares of the other players in the market  for the same services and products for the various categories of the consumers

It is only relevant if your offer/brand/product/service is aimed at certain demographic, otherwise it does not matter much.