Why are Parathas smeared with fat/oil within layers? The difference between plain roti and plain parathas is that parathas have oil in between their layers, and roti's don't. 

Even after we put oil in the layers we still smear the front and back sides with oil while cooking them. That may be for taste though.

What purpose does oil in layers serve?

If we put the same amount of oil in flour while kneading the dough, instead of putting it in layers, will the effect be same?
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.