"I am missing something. Not clear what it is."

Article 1: Section 8 of our constitution gives Congress a monopoly patent on creating the US dollar and also the power to tax. If it taxed first we would all be in jail for non-payment because there would be no dollars to pay the tax with. It must first spend dollars into the private sector in sufficient quantity to satisfy the tax. The tax is never a "revenue source", but only a means to create demand for dollars and to control the total number of dollars in the private sector.

Once collected, the dollars are just tax credits that have served their purpose and are destroyed by the debt (a tracking entity only, not an actual debt that taxpayers are on the hook for) that created them. Money doesn't even exist in the government sector for this reason. All new spending is with brand new dollars created in the private sector under the direction of Congress.

"Creating money is a separate function handled by Central Banks. In USA, Federal Reserve Bank has independence from government. "

The Fed cannot create money and it is very much a creature of Congress and subject to Treasury. Its primary function is to serve as the clearing bank for Treasury payments made in the private sector. Creating money is not a function of "monetary policy. It is central to "fiscal policy" which is the sole responsibility of Congress.

"MMT tries to justify political control of monetary policies. Too much control would be in hands of Federal government and incumbent politicians."

MMT simply offers an accurate description of money and how it moves through the economy from its creation in federal spending to its destruction in taxation. It has no need to "justify" anything as the system currently works as it describes. Realizing that is the best way to monitor and adjust the many factors that make up our economy instead of forcing the system to comply with a number of urban myths and outright lies that have no basis in reality.