It is used to talk about actions which at a given time in the future will be in the past or will have just finished. It is often used with a time expression beginning with "by":

Many daily activities can cause air pollution. If we do not do something quickly, in a few  decades we will have polluted the whole earth´s atmosphere.

  The point of reference is expressed by in a few decades (a time in the future). By that time the action expressed by pollute will be in the past (past in the future).