Bocconcini alla panna di bufala, as described by Regione Campania, are the cherry-sized mozzarella pieces that sit immersed rather than dry on a plate. That bath is the whole point: a small, milky bite already dressed in buffalo cream, meant to be eaten as a complete mouthful instead of a cheese that still needs dressing.
Those same cherries reach shops in plastic tubs. The containers make carrying them away and keeping them simple, so the cream stays with the cheese until you open the lid. It is a practical frame for a perishable dairy, not a claim that every mozzarella in Campania is packed this way.