Because if you are trying to ascertain the relative conditions of living in Russia compared to the USA and somebody uses the price of gasoline as their measure it is necessary to consider how much that gallon of gasoline costs relative to how much money the people in the country have to spend.
That's a basic principle of doing comparisons.
I hope you would agree that if you want to compare the standard of living of the subsistence farmers in the Azores compared to people in the USA you need to consider not just the prices of goods in both countries but how much money people earn.
Nothing in the posts has anything to do with "doing" anything about the differences in economic conditions.
LOL I'm not.