I always assumed that the screw was holding the universe together or something.... But then I took it out, nothing happened and I had to re-think my theory.
I just use a Strat case for my Mustang... There is a bit of space, but it's not too bad. It was really cheap off Thomann (when they were cheap ) and it does the trick.
I bought my Mustang from a local music shop here in Orkney (K2 music if you must know) back in 2006. I got it reduced for £400 instead of like £520. I was chuffed.