how to distinguish the three OGRE OPENGL DX

deal all:
I knew that ogre is a 3D engine,OpenGL and DirectX are renders.
it seems that i never installed OGRE and i have written opengl programme for a while, does it mean an engine is not necessary or an engine is only for optimization?

thanks in advance!

“Engine” is just a fancy word for “library”. Do you need one? No. Could it help you get your actual work done faster? Possibly.