Demographics refer to the characteristics of a specific population of people. They can include a range of characteristics such as age, gender, education level, profession, organization type, employment role, occupation, location, income level, ethnicity and marital status.
First, demographics define the survey audience to be targeted. In other words, they help you decide who to include. Carefully defining the audience will help to ensure you do not leave out important groups of respondents.
Second, depending upon the aims and objectivs of the survey, certain demographic questions can be very important for analysis of the survey results. If you don't include them you may not be able to filter and analyse your survey results as effectively as you wish. For example, if you wish to analyze the perceptions of different age groups then you must include a question with age group options. Or, maybe in an employee survey you need to differentiate the perceptions of different employee levels or roles. If you don't ask for those details then you won't be able to filter results by the different levels or role groups.
Demographic questions should be designed based on the ‘need-to-know’ principle. If you don't need it or it does not add value to the analysis or survey context then don't ask it.