Brunch is a builder. Not a generic task runner, but a specialized tool focusing on the production of a small number of deployment-ready files from a large number of heterogenous development files or trees.
Brunch is fundamentally specialized and geared towards building assets, these files that get used in the end by the runtime platform, usually a web browser. It thus comes pre-equipped with a number of behaviors and features such as concatenation, minification and watching of source files.