There is a grammatical form in the chumash of the vav [hahipuch]1 which turns words written in the past form into future tense and future tense constructs into past. I noted this, I think, pointed out explicitly in the first pasuk of Va'etchanan, where the Avi Ezer supports the reading of the first word as "שתיבת ואתחנן הוא עתיד הנהפך לעבר" (unless there is another grammatical reason to account for the tense change besides the vav).
So I can understand that this construct exists, but I don't yet understand WHY it exists. If there is a way of creating the "right" form of the verb, why write it in the opposite way and then put a vav on to change the tense? If the Ibn Ezra says that it means "וכבר התחננתי" why wouldn't the text simply use the past tense (perfect or pluperfect) in the first place? Is there an opening for a commentary which would be foreclosed if the text simply wrote the appropriate tense?
In cases where the vav hahipuch is used, is there a deeper reason why the tense is written in one way and then inverted?