A unit of money in Country X may buy far more goods in Country Y than in the former. But Country X prides in calling itself a larger economy and a more prosperous entity. As money is just a social construct created as a substitute to represent the value of goods and thus is useless by itself : How can mere money, such as GDP be given more importance and atfention than its PPP counterpart ?
Nominal GDP is often more quoted in media and in politics than the GDP in PPP, but that doesn't make it be less important. In fact, as stated by 1muflon1 on the comments, economists typically like to measure differences between comparable things, so when comparing GDPs there are usual corrections done on purchasing power parity, as well as inflation.
The PPP indicator has been present in literature for a long time, mentioned in the early 1500's, and translated to modern terms since 1918. It's regularly calculated by international bodies (such as the UN under it's International Comparison Programme).
The GDP PPP is also regularly used in real life by institutions such as the OECD, IMF and World Bank to make recommendations on economic policy, and by traders to better assess when stocks and currencies might be under- or over-valued. This article on investopedia provides a concise overview.
GDP is not just mere money. GDP is the monetary value of all finished goods and services made within a country during a specific period.
The importance of GDP comes from its analytical and comparative value:
-It tells us an enormous amount about how a nation is doing. -GDP growth rate measures how fast the economy is growing. -Real GDP per capita describes the standard of living of a country. -The debt to GDP ratio describes whether a country produces enough each year to pay off its national debt. -Economists can use GDP to determine whether an economy is growing or experiencing a recession. -Investors can use GDP to make investments decisions
Where as Purchasing power parity (PPP) is a popular metric used by macroeconomic analysts that compares economic productivity and standards of living between countries.
Some countries adjust their gross domestic product (GDP) figures to reflect PPP
It is used to for: Developing reasonably accurate economic statistics to compare the market conditions of different countries.
To provide insights on the potential overvaluation or undervaluation of a nation’s currency.