Hey Scramble, you seem to be a little mixed up. What difference does it matter why they ask for your ethnicity? The word "demographics"? If you are not sure what it means, then I would suggest you look it up.
There are plenty of reason, that probably could be made into a hub, for asking someone what their ethnicity is while doing a survey or poll or for that fact anything. Especially, when writing is involved in the activity.
The United States of America prides itself on the "melting pot" of different ethnic/racial background of the citizenry. The information is used in more ways than one can count on both hands.