And the reason you'd do this is... to lock in yield in a declining interest rate environment?
A more relevant example is if futures are trading rich relative to bonds - say they are too expensive by 1/32th (about 0.03%). In this case you buy the bonds on repo and sell futures against them, expecting to profit when the price gap closes. Of course, 0.03% is not much profit, so you use 50x leverage (which you can easily do on repo, because it is secured borrowing) turning it into a 1.5% profit.
The five-year US treasury currently yields 1.658%, and SOFR, a measure of repo rate, is currently 1.55%, so that looks a bit like free money. As long as interest rates don't go up.
You might be selling bond futures, and want to cover your position.
You might think some specific bonds are undervalued relative to others, in which case you can buy the undervalued ones on repo, short-sell some overvalued ones, and wait for the market to correct itself.
I'm sure there are many far more ingenious things you can do with bonds.