对话中,如何理解句中出现的现在完成时态,为什么要用现在完成时?

对话中,如何理解句中出现的现在完成时态,为什么要用现在完成时?
Conductor:Fares please!
Man:Trafalgar Square please.
Conductor:I’m sorry,sir.I can’t change a pound note.Haven’t you got any small change?
Man:I’ve got no small change,I’m afraid.
Conductor:I’ll ask some of the passengers.
Conductor:Have you any small change,sir?
1st Passenger:I’m sorry.I’ve got none.
2ndPassenger:I haven’t got any either.
Conductor:Can you change this pound note,madam?
3rd Passenger:I’m afraid I can’t.
4th Passenger:Neither can I.
Conductor:I’m very sorry,sir.You must get off the bus.
　　None of our passengers can change this note.They’re all millionaires!
Two Tramps:Except us,conductor.
1st Tramp:I’ve got some small change.
2nd Tramp:So have I.