What good would an "emergency fund" during "good times" do? What purpose?
It's about the business cycle.
If government runs a surplus in good times, that slows down economic booms, and mitigates (or, hypothetically, eliminates) the crash at the end of the boom.
If government runs a deficit in bad times, that softens recessions.
I don't think in terms of emergency funds, but if you do want to think in those terms, then building up the fund in good times keeps the good times lasting longer, and keeps them from ending as painfully.
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Why don't I think in terms of funds?
Partly, it's because, when we send tax monies to the government, we could accomplish exactly the same thing by burning the money in our front yards. That would have the same anti-inflationary impact, which is one of the reasons for collecting taxes.
Partly, it's because, when we
say there's a fund, it's a bookkeeping trick, a sleight of hand. We put the money in the emergency fund, and then we take it out again, and say we're "borrowing" it to build roads. So the fund consists entirely of a bookkeeping move.
Partly, it's because, when we're supposed to run a deficit because of a depression, we're supposed to do that regardless of whether we behaved ourselves by running a surplus during good times. The "fund" can be empty, but government is still supposed to spend more than we take in; that's how we get back to good times.