Insulation only works if heat must pass through it. Wherever metal, solid plastic or wood connects the warm outside to the cold inside, heat takes that shortcut. Typical examples are door frames, hinge mountings, screws through panels, shelf supports anchored through the wall and metal skins wrapping around panel edges.
Thermal bridges add heat load and create cold spots on the outside surface. In humid conditions these spots sweat or grow mould, which is often the first visible sign. On freezers, bridges around the door opening are why manufacturers fit anti-sweat heaters there.
Good design breaks thermal bridges with plastic thermal breaks, careful joint profiles and fixings that do not pass right through the insulation. For buyers, a cabinet with condensation lines on the outside along joints or around the door is likely suffering from thermal bridging or a damaged seal.