Why do we borrow in our own currency if we can create it?

Treasury bonds (government 'borrowing') serve several purposes that have nothing to do with financing government spending:

First, bonds provide a safe asset for savers. Treasury securities are considered the safest financial assets in the world. Pension funds, insurance companies, foreign governments, and individual savers want a risk-free place to park money. Bonds meet this demand.

Second, bonds help the Fed manage interest rates. When the government spends, it adds reserves to the banking system. Without bonds to absorb these reserves, the overnight lending rate would fall to zero (as it did during quantitative easing). Bond sales drain excess reserves, helping maintain the Fed's target interest rate.

Third, it's tradition. Bond issuance dates back to when the US was on the gold standard and genuinely needed to borrow. The practice continued even after the constraints disappeared. Congress could change the rules, but inertia is powerful.

The sequence is not: collect taxes/sell bonds -> spend money. The sequence is: spend money -> sell bonds/collect taxes. The spending comes first; bond sales and taxes happen afterward for the purposes described above.

Japan demonstrates this clearly. The Bank of Japan owns a huge portion of Japanese government bonds, which it bought by creating yen. The government didn't need to borrow from private markets to spend - the central bank can always buy any bonds the market won't absorb.