The purpose of the shim is to make the vibrations of the barrel be uniform from shot to shot. Think of the shock aborbers on a car, which dampen the springs. (Which is why the Brits use the term "dampers".)
My uncle taught me this little trick when I was a kid. It works. And it's cheaper than a BOSS.
Keep in mind that it's not always necessary, but I've never seen it hurt anything, groupwise...
, Art