I've played tennis twice. + why present perfect?

Hi.
This is such a difficult topic to me. Present Perfect. Can you please explain presnet perfect is used the this context?

I've played tennis twice.
I've been to London twice.

Can't think of any "rule".

thanks a lot


edit: another one. 546 people have died. I saw it on cnn during the tsunami.

ok.
Since I'm going to the US on the weekend for 10 months I am particularly interested in AE

I do know that Americans do use simple past in certain cases in which Brits would use the present perfect whenever Brits use the present perfect because it has an influence on the present situation. For example: You killed a dog. The owner says: What did you do to it. A brit would use : What have you done to it.

So, would an American use the simple past in this context too?

And what about the sentence with the dead people.

567 people have died. Is this also important for the present? Or does it mean that there are still people dying?

thanks

Thats the explanation I was looking for !!!!!!!!! thanks so much