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The feature is to provide an option for the build-all to happen automatically in the background when a change is detected - such as a file save. This feature has existed in Eclipse since forever. If implemented in NetBeans, this may help speed up some tasks such as "run-single", "test-single", which runs way slower in NetBeans as opposed to Eclipse. No proof the speed gain in Eclipse is because of the automatic background build, but having that feature definitely wouldn't hurt.
Reassigning to "java".
Overtake.