The main incentive of the Sitepackage Builder is a quick and easy way to create a new sitepackage, what would be called "theme" in other CMS, for your project. See the Sitepackage Tutorial for details on sitepackages.
Kickstart a TYPO3 Extension with the "Sitepackage Builder"¶
The Sitepackage Builder can also be used to kickstart a basic extension. You can do so by choosing Fluid Styled Content as base package and then remove all files and dependencies that are not needed.