These theories are good to know and some of them might help you out when you feel stuck with some image. Be aware though that they can also take away the originality from your work if you rely too heavily on theory. I've seen people change a strong composition to more generic and boring just because the theory says it's better this way.
Then again if you are a concept artist in a big game company, your consistent art that looks the same as the next guy's might be exactly what they want.
All in all I just wan't to warn you not to rely too heavily in theory and encourage you to trust your own vision for what looks good.
What can you do with composition?
- Guide the eye to some area or point.
- Create more aesthetic images with arrangement of elements
Here is a site that finds phi in everything..
Phi is used to divide the image in to chunks that makes the image pleasing to watch.
This has been used since the building of pyramids.
Theory in practice
Ok, I admit, that theory is a bit thick and math heavy. I think it's better to look at those theories through examples to give you a better understanding what all that means.
The following examples are taken from this site:
I've collected some examples from there, but you should check it out if you want to know more.