Let's talk about the relation of data to automata. Data represents a type of information, and we may have heard that information can be encoded in bits (base 2 numbers). An automaton is either driven by control or data bits depending on the type computing--information processing--that interests us. Data is the proverbial information fluid or material that flows. If the flow is not transformed in any way, think of this as reflection. Reflection is something familiar to us when we look the mirror or when we shine light onto a mirror. The surface of the mirror reflects, and relays, the information without operating on it. In automata theory, we define that which is passed along by defining "the identity function." The identify function takes as input, some data, and just passes that same data long to another place. Just like a mirror passes along incoming light. So data are important for automata--without this concept, the machines would sit idle, the gears rusting.