What purpose, do you think, is there in collected demographic information? Som

 
What purpose, do you think, is there in collected demographic information? Sometimes if grant funded this is needed to report to the funding agency, but outside of that, why collect this information and should one always do so? 
Once you have answered that, please define for us what extraneous variables are and what it means to control for a variable- specifically what a Covariate is (as this is how you control for a variable statistically). How is this related to the demographic information- or is there possibly something else in the study that fits here?