Why are Parathas smeared with fat/oil within layers?
It's the same idea as puff pastry, when the layers of oil/fat heat up they expand and make air pockets in the dough which gives the flaky consistency.

The fat or oil needs to be layered and not amalgamated to get the flaky layers.  If it was spread throughout the dough randomly it would be a more spongy texture.

The other name for this is laminated dough.

In short the layering of dough and fat/oil are to generate texture.