the purpose of TreasuryDirect.gov is to offer individuals access to the very short-term Treasury bills, notes, and bonds. You get to schedule your purchase dates and maturities. A fund offers more liquidity, because you can sell at any time. But in a rising rate environment the yields may lag a bit, as they must wait to reinvest the securities they hold — or invest new money coming in.
And remember, with longer term bonds (10 years or more) you’ll see bond prices fall if rates rise. That means you could lose a bit of your principal if you sell a mutual fund in a period of rising rates.