Capuchin monkeys in Venezuela often rub a certain type of millipede into their fur  Secretions of these millipedes have been shown to contain two chemicals that are potent mosquito repellents, and mosquitoes carry parasites that debilitate the capuchins  The rubbing behavior is rare except during the rainy season, when mosquito populations are at their peak
Ttherefore, the monkeys probably rub the millipedes into their fur because doing so helps protect them against mosquitoes
Whether the only time the millipedes are readily available to the capuchins is during the rainy season