A surplus is not "not doing anything". It's paying off public debt, therefore reducing future interest payment burden.
Not always
A surplus can be earmarked for new projects or spending more on current programs
Best use of surplus is tax rebates
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I’m reminded of Ron Swanson wanting to cut every taxpayer a 50 cent check, when they were debating what to do with a small surplus.
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That's true for governments that carry a debt, but many states have balanced budget amendments If one of those states were running a surplus, that money would not be paying for anything. It would just be taxation for taxation's sake.
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