Water vapour naturally moves from warm, humid air towards cold, dry air. Around a refrigerated space, that means it constantly tries to push into the insulation from outside. If it gets in, it condenses where the foam is cold enough, or freezes in a freezer wall. Wet insulation loses much of its insulating value and can corrode metal skins and fixings.
In factory-made cabinets and panels, the outer steel skin acts as the vapour barrier, as long as joints and penetrations are sealed. Problems arise where pipes, cables or drain lines pass through the wall, or where a panel skin is damaged. Each opening must be sealed on the warm side.
India's humid seasons make this especially important. A small unsealed cable entry on a freezer or cold room can, over months, lead to ice inside the wall, bulging panels and rising electricity bills. During installation and any modification, insist that every penetration is sealed with a suitable sealant.