Im have being wondering a long time.
Why does one needs to check if the string of an OpenGL extension is present using glGetString(GL_EXTENSIONS, when you will anyway need to get the address to the functions of the extension using wglGetProcAddress() which will return NULL when the extension is not present?
Some extensions do not add functions - they only add additional parameters which can be passed to existing functions. If you use these extension but they aren’t supported by a user’s driver/video card then there will be unexpected behaviour in your program and the wglGetProcAddress won’t help because it only lets you know if a function is supported or not.
wglGetProcAddress doesn’t test if a function is available. You should not rely upon it returning NULL if an extension is not available, it could just return any invalid pointer.