What is Present Perfect Tense used for?

The Present Perfect Tense is used for (a) unfinished actions as in I've worked here for three years and (b) finished actions as in 'He's drunk ten cups of tea today. We also use 'since and 'for' in this tense for point of time and 'period', respectively, as in 'I haven't met her since her marriage' and 'I've been hungry for hours.