Frequently, closures are used as completion callbacks for asynchronous operations, such as when dealing with network requests. It is quite common to model these sorts of operations in such a way that an object instance represents a request/response transaction, for example:
When working with escaping Swift closures, it is a common pattern to have the closure capture self weakly to avoid creating an object reference cycle.
Swift protocols are useful for the declaring interfaces that must be provided by conforming entities (in other words: structs, classes and enums). Adopters of a given protocol are free to provide any implementation they wish, so long as it supplies the necessary interfaces.