Just finished replacing my water heater. The old heater came with galvanized nipples with a plastic insert sleeve. The outlet nipple had a plastic ball which looked like a check valve but didn't seal in either direction. (The inlet nipple is rusted in.) A label on the tank said they were "Heat trappers".
The store that sold me the new tank sold me two hollow (no inserts) brass nipples.
But now I'm wondering "what's the difference". What did the plastic sleeves do?