PPP GDP is useful to compare, particularly politically because if the cost of goods in a country is lower people are more likely to be healthy / comfortable with less income. I think it is a little dubious when you use PPP to compare th3e size of two economies because 1) you are taking two approximate measurements and multiplying them 2) countries with lower GDP / person typically have higher relative purchasing power.
In "rich country" (high GDP /person) you can charge a lot for a cup of coffee. In "poorer country" you are likely to charge less. Therefore you can say that person in the poorer country is not as badly off as the absolute numbers suggest. However, if "poorer country" got to the same GDP / person purchasing parity might end up being almost the same. Or, countries with lots of poor regions will have a better PPP, but the cost of living in the places where people have high income may have similar costs.
And in general the most valuable purchasing power differences don't happen for the most valuable and traded goods. An equivalent airplane is going to cost as much in India as the USA, on average. At some point the absolute number is also important.