Why, exactly, does the IRS have fully-automatic assault weapons?

Because they want to use them.  On you.

The GAO looked at how 20 federal agencies spent money on firearms and ammunition, and how those agencies, including the Department of Health and Human Services,  Environmental Protection Agency and the IRS, practiced “inventory control” for their guns and ammunition. But another question raised by the GAO analysis is not answered. Some of those agencies, such as ICE, FBI, US Marshal Service, Secret Service, etc. need to be well armed on the roles of protecting the public. Other agencies such as the IRS and the NIH seem to be over-weaponized, a frightening concept for a government that is supposed to serve the people, not vice versa.

Federal Marshals?  Obviously need guns.  Border Patrol?  I'd give them fully armored vehicles with M2's mounted on a turret to take care of the Coyotes and the cartels.  But the National Institute of Health?

The IRS?  Excuse the hell out of me, but the IRS has already shown it cannot be trusted with any kind of power.  Why the hell does the IRS have fully automatic assault weapons?

Note to grabbers, unlike your false claims about semi-automatic weapons such as AR-15s, what the IRS has are automatic weapons, that’s fifteen machine guns with 56,000 rounds. Who do you think they will be using them on?

This is just another part of the reason why I will never ever ever ever ever give up my guns.  I will not register them.  They all got lost in the Great Canoeing Accident of 2015, so I don't even have them anymore.

The government is force.  Apparently, they want more force.  Just who do you think they'll be applying that force TO?