A possible example of prophecy referring to past events can be Joel's four waves of locusts, unless Joel lived even before the deportation of Israel, which is not entirely consistent with the rest of his book.
There is also the need for revelation of things past, for even if we have evidence, they are incomplete, unreliable, and incorrectly interpreted, at best. For more often than not, history is as silent and hidden as the future. For example what evidence do we have of Joshua's long day or the parting of the Red Sea and the Jordan, or even of King David.
The second reason is to reveal what was happening in the spiritual unseen realm that manifested itself in parallel historical events. An example is the Prince of Persia detaining one of the likeness of man to come to Daniel.
The third reason is to be a witness to the veracity of the prophecy itself. For all things are established true by two or more witnesses and the truth of prophecy is it's foreshadowing. And such a shadow may have unfolded before the prophecy was given. Again Joel's four locusts' waves is a possible example.
But these are mere human reasoning. Where, if any, is the biblical witness for, or against, this principle of interpreting prophecy?