If I asked you to count the number of brushes in the left-hand photo, you'd be able to do so quickly. In the right-hand version of the painting you'd have to spend a little longer and, ultimately, may be uncertain because some of the brushes are hidden behind others.
In these two photos from a work-in-progress, the left photo shows the brushes in the container as I initially painted them. Stepping back a little later to assess what I'd been doing, I realized that I'd made a neat and tidy arrangement: two tall brushes and four shorter, all equally spaced. How boring to look at. One glance and you've taken it all in.
Whereas on the version of the painting on the right, I've added several more brushes of varying heights and angles. It's far more interesting to look at, it engages your attention and keeps you looking for a while, which is what a painting's composition should do. It's the Rule of Odds in action.