DirectX 9 launched alongside Windows XP, the most beloved Microsoft operating system of its time. Crucially, Windows XP could not upgrade to DirectX 10. When Vista arrived in 2007 with DirectX 10 locked to the new OS, developers faced a dilemma.

: Developers use this toolkit to build applications, requiring header files like and libraries for matrix math and geometry 3. Working with DirectX 9 Today

Celebrating 20 Years of DirectX 9 - Microsoft Developer Blogs

Even today, popular indie titles like Terraria and Stardew Valley utilize as their primary renderer because of its low overhead and massive hardware compatibility.