RBI buys USD worth around 5% of GDP

The following is an excerpt from an article in The Hindu:

The U.S. Treasury Department had recently retained India in a watchlist for currency manipulators submitted to the U.S. Congress, citing higher dollar purchases (close to 5% of the gross domestic product) by the Reserve Bank of India (RBI).

Our overall reserves have been fairly steady at $500 bn to $600 bn

Q: why is RBI buying dollars?

I read that it supports building exports and it's used as a reserve in times where inflows would drop dramatically. I didn't understand the second point completely though.

Also, what happens to this reserve if USD($) gets devalued?
(it can benefit US as well right? - US products will be competitive in global markets, they can get some relief on the debt they have as they will be paying money which is less valuable, helps stimulate local economic activities or this could happen due to hyperinflation - been reading about this quite a lot since warren buffet's QnA)