To shore up the FX value of the USD in an emergency,

https://en.m.wikipedia.org/wiki/Foreign_exchange_reserves

Most other countries hold USD for this purpose, but of course the US can't do that.

Note that this has literally nothing to do with the ability to repay debt, and that the Treasury bills market is larger and more liquid than the gold market. The US holds like 600 B$ worth of gold. There are like 30 trillion dollars in outstanding treasuries. Even if all the gold was liquidated at current market value without slippage, it would only be enough to pay back 2% of the debt.