GDP PPP measures something closer to the real value being consumed. It's how big the economy is in a more tangible metric.

Adding per-capita to it answers where people are doing the best, economically, on average. And you're just going to get Luxembourg and Liechtenstein and Norway and Qatar at the top of that list. But a smaller country having a higher average is less significant than the same for a larger country. If you want to see how much clout a country's economy has, and maybe get a proxy for its tax base, you use GDP PPP. If you want to say "my economy could stretch further than your economy", you use GDP PPP.