IMO it is there because: it is light weight, cheap, helps the bumper stay in place when it is really hot, and so that when you re-install you bumper you can forget it on the bench! "Damn! Gotta take the whole stupid thing back off to install the foam."
Edit: I knew I read something a long time ago about doing this...
http://www.ft86club.com/forums/showthread.php?t=29807