I always thought there was an unwritten rule about whether and man gives his number or asks for hers.
Ask for her number
If you are sufficiently familiar with the girl. I.e. know each other though friends, have talked for a long time at a party.
Your confident that they she is interested in you.
Give you own number
When you've only meet the girl for a short period of time, but have got on really well. Your not confident that she is interested in you, she may just be friendly, she may even have a boy friend or like girls.
I was in an opticians and was getting on really well with the girl helping me wear contact lenses for the first time. We had a laugh and were lightly flirting. I was talking to her for half an hour. I wanted to see if she wanted to go on a date. I could have asked her for a telephone number, but if she had a boyfriend this would have caused an awkward situation, that would have change the mood from friendly flirting to a difficult shoe staring. Instead I gave her my number just as I left. It was quick, she was flattered. She never phoned but because it was such a brief encounter I wasn't upset.
It usually means he's to chicken to ask for yours. A guy asking for your number is usually a guy that's far more confident.
Do with that information as you please. Doesn't mean the chicken guy isn't more or less interested than the other guy.but he's probably a little too intimidated to handle your rejection.that is you choose to reject him.
Although I agree a small bit with Deline's answer, I think it's more polite nowadays. With so many nut cases out there and the stalking issue, giving the girl the option to initiate further contact with someone they met in a public place, such as a bar, is a safe practice.
Its just a way to avoid putting the girl in an embarrassing situation that she might shy to say no and give her number while she doesn't like to, so the boy give his number and if she is interested then she will call back, some way of showing high respect to the girl and high dignity
1) In person I don't ask for her number. I ask for her phone. This demonstrates her interest level right on the spot. If she's truly interested in a follow up, she'll hand it over. If she refuses, you walk away. When she hands it over, send yourself a text from her phone. Then give her phone back and reply right there saying "Call you soon" with your name attached. I always follow up by saying "Now you won't mistake me for another wrong number." Women dig the confidence and it lets her know who the call is from.
2) If it's a woman I met online, I always give my number and say "Shoot me a text so I have your number and I'll call you." Once again, it demonstrates her interest level. You're still leading the interaction and planning a date when you call her. But at least she shows she truly wants to talk instead of blowing off your call.