What does glUseProgram(0) do?

Question

The OpenGL docs for glUseProgram claim that calling it with an argument of zero will cause the results of shader execution to be undefined.

However

Answer 1

Once you are using glUseProgram(myShader) all subsequent rendering will be done using the shader ID myShader.

If you are calling this each time you have a material change in a modern openGL 3.1+, there is little use of glUseProgram(0).

However, if you are planning to integrate some fixed-function rendering in there, like glVertex3f(..), then glUseProgram(0) will make sure that the last used shader is not active anymore, and it will use the fixed-function pipeline, like glColor3f(...)