I have no real idea what the marks do for others, although they appear to be alignment marks for something offscreen? I don't use them in my greenscreens.
I could be wrong on the intended use, but I have always liked having them. I use them to track camera movement. Sometimes during interviews etc. the camera op will zoom in, or move slightly. This looks really weird if the background doesn't move when the subject does. So I use the marks as points to motion track, then I apply that result to the background.
If you have a moving camera sometimes, and I must emphasize sometimes, those tracking marks help. The key to any good green screen or color key project is pre-production planning.
All you have to do to find footage with green screen tracking marks is Google green screen test footage for tracking. Select video for the file type and you will have several samples free to download.
http://www.hollywoodcamerawork.us/vfx_sampleclips.html comes up at the top of the list.