Should we always use it? What’s the point? Does it have to do with math?
Those are all questions that I can think of off the bat that one would have when they see that phrase. Now, we have to realize that this is only one of the many techniques in photography (and film) that exist. So the use of this technique is only a small part of what we consider photography.
Let’s go to the definition according to Wikipedia: image should be imagined as divided into nine equal parts by two equally spaced horizontal lines and two equally spaced vertical lines, and that important compositional elements should be placed along these lines or their intersections.
So there really is some math involved. The simple way to put it is that you draw a big hashtag on top of the photograph and then use it as a guide.
Depending on what feeling you want to get in the product is where you center the main subject. I for one use it as a guide to making better compositions, it helps to have a guideline for when you want to have more depth of field in your final image.
Have you ever used the rule of thirds on any of your images? Found out that you did but without trying? (It happens almost naturally) Let us know in the comments below.