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Any Pokemon that uses Rain Dance while holding a Damp Rock will have its duration boosted to eight turns instead of the regular five.
Damp Rock is absolutely vital to get a viable rain team strategy going in the lower tiers, where Drizzle Politoed and Kyogre are banned. By setting up the rain, sweepers such as Swift Swim Kingdra, Gorebyss, and Kabutops become far more powerful, and so dedicated Pokemon with Rain Dance and carrying Damp Rock, such as Bronzong, Uxie, and Crobat, are necessary to provide these optimum conditions and prevent them from being removed. Damp Rock + Rain Dance can also be used in OU as well, as Swift Swim sweepers are illegal if you are also carrying a Drizzle Pokemon.
Route 8: Talk to the Parasol Lady standing just in front of the entrance to Tubeline Bridge between 5am and 10am.