The basis of division is the plate itself. They are just assigning nomenclature to the already existing plates.
Various geological phenomenons can be explained on the basis of plate tectonics. Examples are earth quakes.
Long past, earth underwent a hot unstable phase when there was a lot of transformations taking place in the crust. Later, when it became stable, the crust cooled down, solidified, thereby forming plates.
The formation of mountains is due to plates colliding with each other. Later, earth became more stable and plate movement almost ceased.
But still now, temprature and other variation in the mantle creates minor plate movements thereby causing earth quakes.
Geologic activities such as seismicity (occurence of earthquake), volcanism and mountain formation are the basis of scientists for dividing the Earth's lithosphere into several plates