Sales tax is more usually expressed as Value Added Tax or Goods and Services Tax in countries like Australia however they are normally categorised as consumption taxes. These are "policy" taxes with a view to taxing consumption at the end point of the chain of production (ie GST is a deductible input in the production of goods and services and so it is only the end consumer that actually "pays" the tax). It is not a question of MUST, but a question of the policy goals of the way in which the community is taxed and the simplicity of the overall tax system. There is a need to broaden the base from which government revenue is raised since Australia already has a progressive income tax system and, whilst not exactly Scandinavian in it's brutality, wage earners are already redistributing income down the curve significantly. And the introduction of GST in Australia was accompanied by a rationalisation of a wide variety of other taxes, including Sales tax in order to simplify the base from which non-income taxes were levied.
Even with the "dodged" GST talked about in this report it is much harder for dodgers of GST to dodge it effectively since they get caught on the back end when they go and buy something more "retail" where the GST is just included with no option of omission for cash.
Regarding your analysis of consumption taxes being to hide how much revenue is gathered by obfuscating the means of raising said revenue, that is a crock. Proven so by the detailed breakdown of sources of revenue provided by the Treasury on an annual and longer term basis in terms of post hoc reporting and publicly disclosed projections for policy planning purposes.