Why do we use the Present Perfect Tense?
We use Present Perfect to talk about indefinite past events. We do not know when they happened in the past. There is no exact time reference but there is a connection with the present. We also use it to talk about the experiences and visible results of the past events.
My cousin has finished her homework. It's on teacher's desk now. (We don't know when he finished his homework. It's an indefinite past event.)
I've found my old notebook. (I looked for it in the past and I found it in the past but there is a visible result: It is in my hand, and also there is no time reference.)
I've been to Italy once. (It is an experience I had in the past once. The time period isn't finished: I can still get a chance to see it another time)