difference between OGL & VTK?

what is the difference between OGL and VTK?
I know that a lot of people learn vtk after ogl, why?

For starters, VTK is written using the OpenGL API.

Secondly, VTK sucks for general purpose 3D graphics.

Thirdly, VTK leaks memory left and right with the misuse of even 1 parameter.

Fourthly, VTK is object oriented and complex.

Fifthly, VTK is used primarily for visualization applications (ie: industrial/scientific apps).

And finally, VTK is primarily written for the Unix community - their Win version support sucks bigtime.

Thats all.