The Bible shows us how eternity past affects the future of mankind, and from numerous places in Scriptures we learn that what we have done in the past can and will affect our future. God delivers this message to us in several ways; sometimes He just tells us outright!
He who covers his sins (in the past) will not prosper, (in the present or the future) Proverbs 28:13 (NKJV)
Do not be deceived, God is not mocked; for whatever a man sows, that he will also reap. Galatians 6:7 (NKJV)
If you take a trip through the book of Proverbs; it is full of warnings to us that are known as if/then statements. If you do this that will happen, if you don't do this that will happen.
Have you ever noticed how many stories there are in the Old Testament? There are so many life stories of those very normal human beings who went before us, and their triumphs and tragedies, family messes and battles are all chronicled for us to read thousands of years later. You have heard the saying about hind sight being 20/20; well these narratives are there for us to learn from so we can avoid making the same mistakes in our own lives. We have the benefit of seeing how decisions they made affected them years down the road! Sometimes we see devastating results (Abraham listening to Sarah regarding Hagar, Saul and Samuel, David and Bathsheba, King Solomon) and other times we see glorious results resulting from the past.
For example: because David was able to tell King Saul about the time he saved one of his sheep from a lion and a bear with his slingshot he was permitted to go out after Goliath (1 Sam 17:34-37). In this case, David's past was of great benefit to him and to Israel.
Remembering the past can help us in the present as we are faced with trials and troubles. One only has to look back at Job for this example! Job was able to bear up under the horrible pain and misery of his physical and emotional trial in life because he understood God's faithfulness and his goodness and kindness.
"Shall we indeed accept good from God, and shall we not accept adversity?" (Job 2:10 NKJV)
Without the past to look back on, learn from, and gain wisdom and courage from we would flounder.