Somewhat pointless activities for the most part.
The past is past. Gone to the ether, never to be repeated, relived, or corrected. Just a collection of memories filed away in our heads. Don't get me wrong, I see nothing wrong with happy (or unhappy) memories. But when we habitually dwell on them, they take us out of the present moment and, in effect, rob us of the here and now.
And what of the future? Same thing, but in reverse. Now, is it a good thing to plan ahead and set goals for things? Of course it is. But when we get swept away regularly into a future that hasn't even happened yet, we rob ourselves once again. And let's be honest, projecting into the future can be potentially more detrimental than reliving the past. Because there is no guarantee of tomorrow. At least the fact that I am currently breathing and writing this post is evidence that I have survived the past. But we can't bank on the future. There are just too many variables out of our direct control.
So where does that leave us? Here. Right now. That is all we ever truly have. Now is the only place "everything" truly comes our way.
Master Oogway, the wise old tortoise from DreamWorks' Kung Fu Panda said it best:
"You are too concerned about what was and what will be. There is a saying: yesterday is history, tomorrow is a mystery, but today is a gift. That is why it is called the 'present'."
To your good fortune!