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.

If most money nowadays is calculated digitally (which it is, most transactions are spreadsheets and databases instead of actual cash or goods) and the way most new money that is created is created in similar fashion (it is). Then the constrains of money creation are limited to faith in the system, value speculation (mostly by other countries for what they can trade it for) and inflation. Most monetary systems crash due to overprinting of money and excess inflation which causes a lack of faith in the system. Having something akin to "Gold Sinks" is part of keeping things running, taxes are a perfect way to do that, though the distribution could use some tweaking to get better results, especially if one cares about wealth redistribution.

Not sure where these claims come from either.