Purchasing power only makes sense on a per-capita level; it measures on average what you can by with the sorts of money you can carry. The price will not be the same if the entire country's GDP tried to buy the same goods.
It can also reflect the dynamical nature of the economy in commodities-related industries. A country with higher total purchasing power has higher ability to start and finish projects in low-tech industries like home construction, furniture manufacture and so on.