[Seeker]

Most spark plug spanners/wrenches have a rubber sleeve (bush) inside that grips the plug's porcelain. Plugs tend to be buried quite deep in the head and the rubber bush allows the plug to be held whilst you're pulling the old plug out and stops the plug from falling into the plug "well" when you're trying to put the new one in. They (the sleeve/bush) are a snug fit on the porcelain as a rule.