There was a time, during the early years of the personal computer revolution, when discriminating computing enthusiasts assembled their own systems. If you studied the ads in the the computer magazines for a few months, you could assemble a system more powerful than anything available at retail for the same price. I took some of my first steps as a maker assembling XT and AT class IBM-compatible systems.
Then a sad day came when I realized it was time to stop building my own systems. You see, it was no longer simply a matter of finding the best components for the price and plugging them all together. The complexity of the systems had grown to the point that it took so long to configure and tune a home-brew computer that it wasn't worth the effort (particularly with the price of a perfectly adequate retail system constantly dropping).
In a similar vein, we generally associate symmetry with beauty, particularly when it comes to faces. At least in biology, asymmetry usually indicates genetic defects or disease.
Another way to come at this idea is in terms of proportion. Something is said to be in proportion if no one part overwhelms the whole. This is easy to understand if you've ever heard a volunteer choir with someone whose voice doesn't blend.
Writers show their mastery of balance and proportion by skillfully blending narrative and dialog, through their understanding of pacing, and by including only those parts--from specific language choices to carefully presenting only those scenes that further the story--that contribute to the whole.
When the thing you're making is balanced--when each part contributes in proportion--then you can have a whole that is greater than the sum of its parts. Then you've created a thing of beauty.
Image: Bill Longshaw / FreeDigitalPhotos.net