为什么SDL和OpenGL是相关的?

I was messing around with SDL and found out that you cannot rotate images with SDL. Everywhere the question was asked, people said to use OpenGL to do rotation. I always thought that SDL was completely separate from OpenGL, am I wrong in thinking this? I even found tutorials for using OpenGL within SDL, which confused me even further. What exactly is the relationship between SDL and OpenGL? Why not just use OpenGL if its more powerful and allows you to do more (this is from what I've read)?I was messing around with SDL and found out tha