In a nutshell, the task lets you reference other build files. This means that you can create common centralized libraries of build files that other people can use on their own projects - all without copy and paste. And believe it or not, the semantics all make sense too. You can provide default tasks and properties, and the importer can override tasks and properties to customize behaviors on a case by case basis if it's required. The end result is that individual project build files are smaller and easier to understand, and common behavior can be achieved across an entire large system in a natural and non-cut-n-pasty manner (I don't know about you, but I always found pasties rather unnatural)."
Also new is macrodef: "Macrodef is a way to define a new Ant task in an Ant build itself. Macrodef allows you to define standard tasks that have attributes and elements given to them when they are called."