Also what's the benefit of having a game engine and then a seperate rendering engine? Thanks!
The rendering engine is what shows the game, if you aren't sure about that yet.
Making a rendering engine isn't nearly as easy as you think, and I don't know enough about the subject to stress this as much as it probably should be stressed. It's much, much easier and will save you a lot more trouble to use libraries that are already functional with extensive documentation such as OpenGL and DirectX.
If I were more confident that it were true, I'd also say that graphics card drivers have to be made to support rendering engines. Sounds like something I pulled out of thin air, though, so I guess I'm really just asking or something by saying this.