[twic]

There are as many reasons to do it as there are reasons to buy bonds!

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.