Because the tutorials were compiled on windows, and the GL headers (in MSVC at least) have this annoying quirk that they use some #defines from windows.h.

Microsoft''s OpenGL implementation uses GDI macros in the GL/gl.h header and requires that they be defined before the header is included, hence the need to include windows.h before GL/gl.h. Since it''s only that OpenGL implementation that requires that, feel free to wrap the windows.h inclusion in a preprocessor test to only do it when necessary (less global-namespace-poluting solutions exist too, by avoiding the need for windows.h, but they''re more difficult).