I'll preface this by saying that I'm sure someone else has done this already, so apologies if this has come up before on Pinside. The Pinball Buddy serves one purpose only: it holds up the back (or front) of your game while you install or remove the legs. It's a portable, fairly lightweight alternative to a bar stool. I'll start with a quick picture of the two that I'm making for SFGE (work in progress):
Behind them, you can see the bulky wooden version that I made about 10 years ago. It has served me well, but I needed a few more so people can help me set up and tear down games at SFGE.