They want to know so they know what box to check for you.
Got it. A sea of unofficial census-takers. That's what I thought.
How irritating.
ETA: I'm not saying I'm above wondering or something (I remember being like "ah, I see that" when I learned one of Olivia Munn's grandparents was Chinese), I just wouldn't ever ASK. Like, what am I going to do with that information other than know it?
in addition to checking a box, I think, for some people, they feel more comfortable knowing what category others fall in b/c then they have a better idea of how to interact with them. you know, b/c everyone from the same race/ethnicity acts the same and likes the same things.
I used to get asked a lot if I was "Spanish". I have dark hair and dark eyes. One sentence of my mediocre Spanish would quickly disabuse anybody of that idea.
Coming from the midwest where if you called someone who identified as Latino or Chicano "Spanish" you need to have your running shoes on, I can't tell you how shocking it was to move to New York and find it was no big deal for folks to call other folks "Spanish" and they didn't mean from Spain.