Have you ever needed to load a VERY large file in ruby? No I don't mean your 500 line rails model. I mean a several gig binary or text file.
Try it. Watch your machine become unresponsive. Cry a little inside. Then reach for enumerable.
Ruby's Enumerable module gives you a way of iterating over collections in a lazy manner, loading only what you need, when you need it. But it gives us so much more than that.
rubylove has written a great, in-depth look into enumerable with: A Deeper Look at Ruby's Enumerable