Nothing I've purchased has ever made me a better photographer. Many things I've purchased have inspired me to become a better photographer.
OK, that was the right answer, but also the cute one, and I know you want a little meat to go with it. First off, I'm going to relate this only to the taking of the picture, which for me is where
photography is concentrated. There are many things I've purchased that have immediately made me a better producer of photographs - but that's a different thing (we can go there later if you want).
There are two ways to improvement for me that would relate directly to adding new equipment. First, I am finding a limitation with the equipment I have and need something else to express myself as a photographer. Second, the equipment opens a creative door for me that I didn't know was there or that I wanted to explore.
Both of these happened for me when I got my Sigma 8-16mm. I'd lived life with an 18-105mm and a 28-300mm, and a few primes, but as I looked at the work of folks that inspired me I knew there was something about
WIDE that I wanted to do and couldn't. The minute I put it on my camera I was able to grab hold of that vision. But at the same time, after seeing the world from that perspective I was never able to look at it again the same way. Pretty (boring) landscapes now became these huge vistas when the light and the clouds were right. Corners of a building now became these divided scenes open for exploration. Once I saw them I couldn't stop seeing them, and any time your eye is looking for a photograph where it hadn't before you're a better photographer.