GDP (PPP) is only the indicator that says what is the purchasing power of the SAME dollar spent in say US or say India.
But it is not a good indicator to compare a country Globally.
As you know b'coz of trade, investments etc. in other countries, PPP doesn't matter (as it is for domestic use only).
So, for making a level playing field, we use Nominal GDP.
BTW, i want to add one more point to the real GDP. If we have lesser inflation than last year or say we have a deflation, than real GDP growth rate can be greater than the nominal GDP growth rate. But as we know the case is (almost) always reverse, so we normally have GDP nominal rate > GDP real rate.