Plates as in \'sheets that cover\', more or less.
While I can\'t neccissarily say why THEY used them, I can explain my own usage: While smaller scales are very useful at the joints and regions of muscle movement, large plates are going to be better at defense, and won\'t hinder you if place on the few areas of the body that have little movement (essentially, the parts of the limbs where you generally have very little between skin and bone, meaning that there\'s not much muscle bulge).
The design tends to stem from various sorts of more modern style leather armor (seen in sci fi and such), though you can also see the like in some clothing styles I\'ve seen, usually for bikers and such.
If nothing else, picture, if you will, the difference between a snake\'s underbelly and the rest of its surfaces.