A lot of it was a backstop in case it was needed, but those reserves were not tapped.

Basically the Fed made lines of credit (with strings attached) open to banks to guarantee liquidity, increasing trust in markets. Banks didn't withdraw the money, and some that did (or were forced to take it to dissuade bank runs on the others) paid it back quickly to get out from under the strings.

Here's [1] a decent source on the finances of the bailout (which were actually loans).

[1] https://projects.propublica.org/bailout/