I suspect that the word that is causing you trouble is "purchase". "Purchase" here does not mean "the act of buying", but rather definition 2, "a hold or position". That is, you can say, "As he climbed the mountain, he found a purchase for his foot in a small crevice", or "I got a firm purchase against the wall to help me push the heavy piece of furniture." (Probably doubly tricky here because the fact that we're discussing economics likely makes you think of "purchase" in the sense of buying.)
"Purchase on reality" is a somewhat common phrase meaning a hold on real life, that is, an understanding of what is actually happening around you, as opposed to a delusion. We say that someone going insane has "lost his purchase on reality", like a mountain climber could lose his footing and be in danger of falling off the cliff. The phrase is often used in the negative this was "lost his purchase on reality", "has no purchase on reality", etc, to describe some person or school of thought that we believe is not practical or realistic.
So here, the writer is saying that "growth economics" was becoming unrealistic or impractical, but now something has happened to give it a practical purpose. (I presume that what that "something" is is described in context.)