A system reliant only on sales tax would do the following:
1- Tax all people fairly, including those who earn money illegally (drug dealers and other criminals) or "off-the books" (migrant or other laborers).
2- Encourage savings, by taxing people on what they spend rather than on what they earn, inherit or otherwise gain.
3- Simplify the tax and audit system, focusing solely on the collection of taxes through businesses rather than individuals.
While these are broad generalizations of the benefits of a sales tax-based system, there are several more.