Tony's right
While we have been hearing in the past few days that people who put money in a bank are essentially "sticking it in a mattress," the money immediately goes back out into investments. Don't know what banks invest in now...it was pretty straightforward when I was younger. They invested in their communities. A savings and loan did just that: took savings in and loaned it out as money for new houses.
Banks ideally keep a sufficient reserve to shield against problems (the business relies on every saver not needing their money at the same time), but I don't know what the requirements are nowadays.