Before OpenGL 3.0, the API was cluttered with legacy functions (like glBegin and glEnd ). Developers were stuck in the "fixed-function pipeline," where the GPU did most of the math for you, limiting what you could achieve.
OpenGL 3.3 is the version where OpenGL grew up. It is the boundary between the "old way" (fixed-function pipeline) and the "new way" (programmable pipeline). If you are writing a graphics engine today and do not need specific hardware features from Vulkan or DirectX 12, OpenGL 3.3 is arguably the best starting point. open gl 3.3