What is the difference between these two sentences?
Danny finished his homework.
Danny has finished his homework.
It is used to refer to a subject's past actions or states while keeping the subject in a present state of reference or in a present state of mind. Think of the words in the construction separately: "have" (or "has") is in the present, and the past participle is in the past.
The boy saw the car. (Emphasis is on the fact that the boy saw the car.)
The boy has seen the car. (Emphasis is on the present state of the boy, resulting from the fact that he saw the car.)
In summary, both the present perfect tense and simple past tense are used for past actions or states, but the present perfect describes the present state of the subject as a result of a past action or state (i.e., the subject is being talked about in the present), whereas the simple past describes solely a past action or state of the subject (i.e., the subject is being talked about in the past).
In other words, it places the subject in the result phase of the event.