By painting over your painting I mean improving on what you have already painted, not painting a completely new painting.
You should paint over your painting whenever you know you have grown as a painter and you realize how much better your painting can become.
Caveat--this is only recommended with acrylics and not oil, watercolor, or pastel without due consideration since those mediums do not lend themselves to over-painting as easily as acrylics.
Anyway, here's the scenario. You're straightening up (or whatever) your studio and you run across an old painting you did way-back-when. You now see it with a more objective and much better informed eye.
"Why did I paint it like that?"
Good news and redemption. Since then, you have put in more hours painting. Since then you've read and watched more instructive how-to's. Since then you have been to more museums, galleries, art shows and exhibits.
In short, you have grown as a painter, and you can improve upon it, and as painters, we all want to improve.
Today images are my before and after paintings.