I have to admit, dh (Ethiopian) and I (white American) will sometimes ask. Not to be snarky or anything like that. We can both spot people who are from Africa and usually their children in a crowd. We are intrested because families like ours are unusual and we are always looking for people who can relate to us and understand the particular issues we face as a mixed-race and cross-cultural family. Now, if it's me asking, and my dh and kids arent around, you might just assume I'm just a white ignoramus who's spouting nonsense for no reason.
For dh it's a sense of "Ah, someone who knows what it's like to be an African in America. Maybe we can relate!!!!! Stay right here, I'm going to see if I can catch them and "friend" them". I did once outdo him by spotting an Ethiopian family at the museum when he didn't. He wanted me to explain, and I could only say that I "just knew", because that's what it was. We went on to enjoy some good times with that family.
The last time someone aske me where my kids were from, I was really glad she did, because it turned into a great conversation. She was a white, middle-class, middle-aged lady, curator at a small local art museum. She was *so* excited by my positive response to her, because, as she told me, her son was getting ready to propose to his girlfriend who was from East Africa and she was soooooo excited about it. Not many people in this area would understand that excitement. So we had a great conversation about that area of the continent and how awesome and beautiful it is etc, etc. If I'd snarked at her we would have missed out, and I'm glad I didn't.