One reason demographic variables such as age, family size, family life cycle, gender, income, occupation, education, religion, race, generation, nationality, and social class are so popular with marketers is that they’re often associated with consumer needs and wants.
Another is that they’re easy to measure. Even when we describe the target market in non-demographic terms (say, by personality type), we may need the link back to demographic characteristics in order to estimate the size of the market and the media we should use to reach it efficiently.