There are problems with PPP but it does try to address some real (no pun intended) issues with nominal GDP.

A US resident might be able to afford a lot of Chinese goods and services but can't get many of them. For a Chinese consumer, the average price of goods or services is less relevant than the price of the subset of goods and services that are actually accessible to them.

It attempts to address the very real problem that a Chinese resident can buy a Chang Li truck for around $2,000 but an American consumer spends over $9000 once the realities of international shipping are factored in.

A theoretical economist might abstract these differences away and just claim that arbitrage will eventually balance out these price differences but empirically we can see that's not what actually happens.