This event is emitted before the builder decides to build all sources. It's most useful for preprocessing sources on the file system before the rest of the build process should pick it up. For instance here a plugin could inject additional files into the asset folder and similar things.
As an example, this is what the Webpack Support uses.
import subprocess def on_before_build_all(self, builder, **extra): root = ospath.join(self.env.root_path, 'webpack') subprocess.Popen(['webpack'], cwd=root).wait()
A list of some plugins that make use of this event is available here.