As lead-acid batteries age, positive plate growth occurs which causes the positive terminal post to move upward. If the terminal post is affixed to the battery cell cover, this growth will push the cover upward. This can eventually ruin the battery if it cracks the cover or breaks the seal.
To prevent this from occurring and to ensure a longer battery life, some battery manufacturers will install a rubber floating bushing instead of welding the terminal post to the battery cover. The floating bushing prevents the positive plate growth from damaging the battery cover by allowing the terminal post to move upward.
The rubber bushing maintains the seal against both the battery cover and the terminal post when positive plate growth occurs.