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If the holder uses Sunny Day, then it will last for eight turns, instead of the usual five. This also works even after the holder switches out.
Since the only Pokemon that can bring about permanent bright sunlight is Uber, Heat Rock is an essential item for Sunny Day teams. It increases the duration of the sunlight from 5 turns to 8, making the setup a lot more worthwhile, as well as allowing the player to pull off a sweep a lot easier.
Pokemon that are attempting to sweep using sunshine related abilities such as Chlorophyll or Solar Power are generally better off using an item to boost their own offensive power, such as Life Orb, instead of Heat Rock, even if they do carry Sunny Day as a pinch support move. Heat Rock is of most use to bulkier Pokemon that are specifically assigned Sunny Day replenishing roles on a team, as these Pokemon will get the opportunity to set up the sunshine more often and usually more than once over the course of a battle.