New Economic Perspectives has the article Randy Wray: What are Taxes For? The MMT Approach.

But in the case of a government that issues its own sovereign currency without a promise to convert at a fixed value to gold or foreign currency (that is, the government “floats” its currency), we need to think about the role of taxes in an entirely different way. Taxes are not needed to “pay for” government spending. Further, the logic is reversed: government must spend (or lend) the currency into the economy before taxpayers can pay taxes in the form of the currency. Spend first, tax later is the logical sequence.

Some who hear this for the first time jump to the question: “Well, why not just eliminate taxes altogether?” There are several reasons. First—as we said last time–it is the tax that “drives” the currency. If we eliminated the tax, people probably would not immediately abandon use of the currency, but the main driver for its use would be gone.

Further, the second reason to have taxes is to reduce aggregate demand.

I have read about this aspect of MMT before, but this is the first time I have read these details and this emphasis.  It helps to hear it explained this way.  It also helps to put up front what are not the implications of MMT that one might  assume having heard only the first part.

The part of needing to have taxes to reduce aggregate demand implies that MMT proponents do believe in keeping inflation under control by having enough taxes to reduce aggregate demand if that demand would become inflationary.  So it is unfair to claim that MMT proposes to print all the money you want so much so that it would cause inflation.  MMT wants you to have the correct understanding of the role of currency for a government that issues its own sovereign currency.  It wants to free you of the idea that taxes need to be levied in order to fund federal government expenditures.