In aggregate, to offset, to the extent viewed desirable, the money supply effects of the creation of money in spending, and in terms of specific distribution of taxes to effect a desired (re)distribution of net income.

Also, though the above would be sufficient on its own, because prior to the abandonment of commodity-representational currency for pure fiat, the metaphor of the fisc, a finite purse (from which we get the adjective “fiscal”) was a reasonable model of government finances, and there is a lot of inertia in popular models of the world. So, even though it is no longer an apt model, lots of people — both in the electorate and even in government — still view the world through a framework in which that is exactly how things work, which affects their decisions as electors or government decision-makers.