GDP (PPP) is calculated by GDP(local currency)*x*y
where x= 1/(exchange rate)
und y= some multiplication factor.
Now what is y
suppose a bag of rice costs 50 Rs in india but 2 € in Germany.
Now direct conversion gives the costs respectively as 50 Rs and 2*70 = 140 Rs. So does rice bag cost double in germany ???? or conversly must a german work double to earn a bag of rice?
Not really. Here depending on the cost of living & the cost of essentials a factor y will be calculated that will make these costs similar.
So assume the factor is 5 (i.e. I can have the same standard of living in India with a one fifth salary(in doller terms) i.e. I will work for 200$ but an american IN AMERICA will do the same in 1000$ )
Advantages:-
Good for comparing markets and Human Development and their moneys strength IN ITS OWN MARKET)
Now,
India goes to the oil market and wants to buy a barrel of Oil. It needs 53$ as of today.
Here you will have to shell out 53*63(exchange rate) = 3393 Rs .... you can slap that PPP in the face of that arab sheikh but it wont help ya one bit.
Same if you go on a trip to US. All that PPP will be like thin air