For example, in the Play framework, they involve Futures so that asynchronous processing can be performed.
Because iteratees are called by other code which feeds them with data, they are an example of inversion of control.
Because the enumerator knows exactly when the iteratee has finished reading data, it will do the resource cleanup (such as closing a file) at exactly the right time – neither too early nor too late.
Iteratees and enumerators combine the high-level functional programming benefits of lazy I/O, with the ability to control resources and low-level details where necessary afforded by C-style I/O.
[5] Iteratees are used in the Play framework to push data out to long-running Comet and WebSocket connections to web browsers.
This makes it easy to repurpose an iteratee to work on an in-memory dataset instead of data flowing in from the network.
In a sense, a distant predecessor of the notion of an enumerator pushing data into a chain of one or more iteratees, was the pipeline concept in operating systems.