food-scienceindian-cuisine
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?
I think it is mostly preferential as you can toast either way. Using oil can be tricky though as there is a fine line between toasting with oil and frying your spices. An article I found boasts using no oil which I like. There is also a list of spices that benefit from Spice Toasting. I do believe toasting with oil can change or influence the flavor depending on the oil used.
There has been quite a bit of confusion here concerning maida (which can be labeled all-purpose, especially in India). The top answer on the question that you linked to was actually incorrect. It has been fixed. Maida is actually a very low protein flour, much like what would be called cake flour in the US. I suspect and would appreciate if you can confirm that your use of the term "all-purpose flour" stems from the Indian use of the term, not that your recipes call for what Americans would call "all-purpose flour".
In answer to your question, yes, you can substitute what you know as white flour and what I know as all-purpose flour (`10.5% protein) for maida by replacing some of the white flour with cornstarch or potato starch. The best reference I could could find for the percentage of protein in maida was The Fresh Loaf, they say that, on average, maida contains 7.5% protein. To get from 10.5% protein to 7.5% protein, you would need to "lose" 3 grams of protein per every 100 grams of flour. By replacing 28.6 grams of white flour with potato starch or cornstarch, your mixture would have the the 7.5% protein content of maida. That looks good on paper, but my gut says to split the difference. I'd first try a mixture of 15 grams of potato or corn starch and 85 grams of white flour as a substitute for 100 grams of maida. The use of cornstarch in that way is commonly done in the US with our all purpose flour to emulate our low protein cake flour. cake flour with corn starch
In case the international flour terms aren't complicated enough, I understand that in Israel what is generally called cake flour has added leavening agents, it's like what I would call self-rising flour. Confused yet? I'll repeat here what I said in comments above:
"It's confusing. In India (and therefore also on its exports) maida is sometimes labeled "all-purpose", but it is more like what Americans call cake flour. Israel has its all-purpose, which is less processed than the all-purpose in the US. What is called cake flour in Isreal would be called self-rising flour in the US. What Isreal has that is the closest to what Americans call all-purpose is called white flour."
Best Answer
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.