I don't think sfml supports creating 3 dimensional games by itself. If you really want to use it, I believe there's a way to use OpenGL from within it, and create 3d things from there, but you'd be better off using something else. Like ogre3d or maybe irrlicht.
I would also recommend Ogre or Irrlicht (or something else -- but those are tried-and-true renderers), and not even because 3D graphics is hard (it is, but IMO it's the fun kind of hard), but purely because storing, loading and operating on meshes is a massive PITA and game development is difficult enough without going through all that stuff. Better to complete a few projects using higher-level libraries like Ogre and then learn the low-level stuff like OpenGL and the various mesh file formats and representations later.
I think this is just a matter of efficiency - you are doing the calculations on the CPU, while you could alternately use hardware accelerated graphics by moving the calculations onto the GPU, which would be much more efficient (and probably more accurate, too).
Well, efficiency is one thing and accuracy is another, as NT3 pointed out, but also, that projection equation doesn't include perspective so different objects will appear to be the same size regardless of distance from the viewer.
As for "you gotta walk before you can run", I disagree. Learning to do those calculations on the CPU before the GPU doesn't really help you in any way.