Bird baths provide an essential water source for wild birds. Providing them with water for hydration as well as bathing in, especially during the warmer, dry months when natural water sources might be scarce. Bathing helps remove dust, dirt, & parasites from their feathers.
Birds also use bird baths to quench their thirst. Flying requires a lot of energy, so having water nearby is crucial for them to stay hydrated & maintain their activities.