Last week alone, the Fed bought another $28 billion in treasuries while continuing to sell mortgage-backed securities on net. Because it's mainly about deficit-funding, not just liquidity. Excess UST supply + dollar shortage = they will create dollars to buy UST for a long time. 
For years, large U.S. banks were drawing down cash levels (15% to 7% of assets in last 5yr) to buy treasuries (from 15% to 21% of assets). But now they are basically at regulatory limits; can't draw down cash to buy more treasuries. So, the Fed became the new buyer of them.
Zooming in on that last chart, large bank cash levels hit lows of just 7.25% of assets the same week of the repo spike and Fed intervention in the repo market. They rebounded back up to 8.26% since the Fed became the relief buyer of treasuries and banks were off the hook.
A lot of people think the Fed is trying to support the equity market, or covering for a bank/fund failure somewhere. Those explanations are possible, but not required. The Fed simply taking over as the primary financier of U.S. deficits is a mathematically sufficient reason.
And keep in mind that foreign holdings of UST peaked in August, and slid down in Sept/Oct, around the time of the repo spike. The treasury needed a new buyer of their debt, and it was the Fed.
Plus, as I noted back in September, U.S. corporations drew down cash and t-bill levels in 2019 to pay for buybacks, after repatriating a bunch of cash in 2018. So that was another UST net buyer lost, that the Fed had to start covering for by buying UST.
