A recent article on Vox discusses the hows and whys of the sculpted Greek bronze cuirass.
After all, it was about looking good...
Possible armor decoration and embellishments:
1. Fantastic abs
2. Fleur de lis everywhere
3. Misc. animal heads
5. Patchwork or motley (birdcage on helmet extra-cost option)
6. Astronomical signs: stars. Moons comets, etc
7. Tiger stripes, rawr!
8. Dazzle camo
9. Leaves and flowers
10. Insane, tacky rococo
12. Indecipherable script (translation/significance TBD...)
13. Battle scenes
14. Cherubs n' shit
1 in 20 chance that the embellishments impart a special quality upon the armor (80% positive, 20% liability)