Is it just the Jewish part that you don't get? The US has so many different active cultures going on in the same spaces that knowing someone's ethnic background can tell you a lot about them and their family. I'm sure some people want to know because they're racist, but for most people it's just a cultural shorthand. Knowing someone is Cuban rather than Puerto Rican, or half Spanish and half Irish tells you what kinds of experiences they might have had, what comfort foods they're likely to eat, how they're likely to celebrate their holidays. Stuff like that. Especially if one of their cultural identities is one that you share, or frequently share the same spaces with, you've probably just found a whole lot of commonalities with that person. Older people might ask. In my experience younger people generally won't. So either it's obvious to you or they tell you or you might not know at all.
From a governmental standpoint, they keep track of different statistics based on ethnicity, supposedly so they can make sure they're not failing any groups of people with representation, healthcare outcomes, policing, etc. It obviously doesn't always work, but that's supposed to be why the government is interested.
Most of us thought similar things about the Capitol. Then January 6th happened. Now, not so much.