A lure was added to Pokémon Go during the Luminous Legends X event, during the Season of Legends. It’s called the Rainy Lure, and it attracts Pokémon that prefer the rainy weather. You can use this device by attaching it to a Pokéstop, and have it remain active for 30 minutes or 3 hours for special events.
The Rainy Lure attracts three types of Pokémon: Bug, Electric, and Water-types. If you’re looking to capture any of these Pokémon, you can find any of them hanging around a Pokéstop that has this activated. It’s a good way to capture those three types of Pokémon through special events, or if you’re trying to hunt one in the middle of a Community Day. The situation can vary, but they’re good to have, and they’re a little rare.
When you want to use it on a Pokéstop, click on the small icon at the top of it and activate the Rainy Lure from your bag. These are a little difficult to acquire, but they’re worth it if you’re hunting specific Pokémon.
A Rainy Lure may also be a requirement for you to evolve a Pokémon. For example, during the Luminous Legends X event, Goomy is making its debut, and one way to evolve this Dragon-type Pokémon is to have it in the presence of a Pokéstop using a Rainy Lure. Alternatively, Goomy can also be evolved by waiting for a rainy day, but Pokémon Go continues, other Pokémon may require a Rainy Lure.