This is basically exactly the point. It's incredibly difficult to predict building 'large software'. So, lets try a few things to help make it a little bit more predictable by trying to predict smaller increments.
> Why do we need predictable deliveries?
Say I'm building a piece of software - maybe an app the audience would use - for the Superbowl. It's very handy to be able to estimate our progress to see if we'll actually land that date. The Superbowl isn't going to move because of our app, and our app isn't very useful the month after the Superbowl.