I have always wondered about them, as I can't see how you know if the weight needs to be on the inside or outside... but it was the way it was done before these new fangled electronic things...
The first set of BFG's I had on my Vitara were too big for the electronic balancing machine, so the old boy in the tyre place came over and showed the young whipper snapper how it was done in the old days on a balls eye one, and they were perfect...
Exatomondo. My son works in a garage and fits tyres most days. They had an eye ball balancer until recently and confirms they are too basic to bother using, let alone buy one
Avoid.