Metal door frames and the vertical bars between doors (mullions) are cooled by the cabinet behind them. If their outside surface drops below the room's dew point, moisture condenses on them, drips onto the floor and can freeze the gasket to the frame on a freezer. An anti-sweat heater is a resistance wire or strip that keeps those surfaces just warm enough to stay dry.
In humid Indian conditions, especially during the monsoon and in coastal cities, these heaters matter. Without them, freezer doors may stick, gaskets may tear and puddles can form in front of the unit, which is a slip hazard and a hygiene issue.
The heaters use electricity continuously, so better designs control them by humidity or run them in pulses rather than at full power all the time. If condensation appears on a frame that was previously dry, the heater may have failed; if the frame feels hot to touch, the control may be stuck on.