Adding to Ruthi's response, "have been" is also used when introducing your experience of something, such as "I have been a volunteer", "I have been here before", or Ronnie's example "I have been to Hawaii".
We cannot use past tense, because we are referring to a time period [from the past up to the present]. Only present perfect can connect the past to the present. Past tenses stay in the past.
One more thing - if the verb represents a "state of being", then you don't use a continuous form. You can say "we have been friends for fifteen years" or "I've known them since I was a child" (it started in the past and continues to the present), but "we have been being" / "I've been knowing" are simply incorrect and awkward.