From Wikipedia:
The present perfectpresent perfect in English is used chiefly for completed past actions or events, when it is understood that it is the present result of the events that is focused upon, rather than the moment of completion. Wikipedia
I make an effort to explain it as following but not sure I am right.
The present perfect in English is used to express an action or event that happened in the past & completed at the present time, but we don't care the time that action / event is completed.
