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Deals damage. Removes all layers of entry hazards (ie. Stealth Rock, Spikes, and Toxic Spikes) from the user's side of the field, and also removes the trapping effect of moves such as Fire Spin and Wrap. It also removes Leech Seed from the user. If the user faints from Iron Barbs, Rough Skin, or Rocky Helmet damage when this move is used, entry hazards are not removed. If the target of this move is a Ghost-type, then the move will fail to have an effect.
Every competitive battler will agree that Rapid Spin is one of the most critical support moves in the entire game. You might ask "Why?" The answer is simple: Rapid Spin removes all of the entry hazards from the user's side of the field. In a metagame where Stealth Rock, Spikes, Toxic Spikes, and Leech Seed run rampant, the ability to remove them all in a single turn is an extreme gamebreaker. Removing entry hazards from your side of the field ensures that low health Pokemon can enter the battlefield without fainting immediately, Pokemon that are weak to Stealth Rock, Volcarona in particular, aren't crippled almost irreparably upon entry, and Toxic poison won't be spread around your team from Toxic Spikes. The utility and possibilities of the move are almost endless, and you'll find a Rapid Spin user on nearly every competitive Pokemon team.
The use of Rapid Spin is so widespread, in fact, that a whole strategy has evolved to counter it. The term is called spinblocking, and the only Pokemon that are capable of doing so are Ghost-types. In fact, spinblocking is almost the sole reason why you'll even see Ghost-types on the opposing team. This is especially true of stall teams, which rely on passive and entry hazard damage to deal the majority of their damage against an opposing team and therefore must keep entry hazards down at all times.