- Because it was normally a hint about their ethnic background or family history, which could lead to interesting conversation or tell me a little more about her.
- Because there was an empty field for it in my phone contact and I felt compelled to complete it.
I suspect other guys have had similar experiences, so it is fair to say that most men won't mind if you don't give them your last name - this is expected. Certainly they won't hold it against you if they ask for it and you tell them "I'll tell you when we get to know each other a little better."
However, on rare occasions I will ask a girl for her number and she will actually volunteer her last name. This seems like a small thing, but in retrospect it goes a long way in forming my initial impression of a girl. Again, I would never hold it against her if she didn't want to give me her last name, but her offering it is like a breath of fresh air. In the middle of a world of skepticism, mistrust, and concern about stalking, you suddenly stumble upon someone who trusts, who is open, candid, and unassuming. Some might call these girls naive, but from the perspective of a guy who is genuinely interested in getting to know her, it is a sign of goodwill, openness and trust. This is very endearing and feminine. People tend to expect from others what they are disposed to themselves, so a girl who is willing to give you her last name is the kind of girl that assumes the best of everyone - probably because she is a good person herself. This is the kind of girl that men like.
So the next time he is taking down your number, offer your last name too.