Banks are required by law to keep a portion of their assets on deposit in reserve. So if a bank has say, 1 billion in deposits, they might be required to maintain a balance with the Fed of 100 million.
What I think you're asking though is "why don't they just not keep the cash at the federal reserve". And the answer is that that's just not a thing you can do. Your bank maintains an account with the Fed - that's how it actually "stores" money. It can also physically store cash, but that comes with carrying costs (protecting it, etc.). Those carrying costs are one form of a negative interest rate.
At the end of the day, someone is storing the money somewhere. And the root-level money storer in the economy is the Fed. If the Fed wants to charge you 0.5%/year to store your money, what are your alternatives? You could keep physical cash. But then you have to protect it. You could invest in other assets, like stocks/bonds/mortgages, but those have risks. If you can't find any investments you like, and you don't want to physically store the cash, you don't really have any other choice.
Banks are required by law to keep a portion of their assets on deposit in reserve. So if a bank has say, 1 billion in deposits, they might be required to maintain a balance with the Fed of 100 million.
What I think you're asking though is "why don't they just not keep the cash at the federal reserve". And the answer is that that's just not a thing you can do. Your bank maintains an account with the Fed - that's how it actually "stores" money. It can also physically store cash, but that comes with carrying costs (protecting it, etc.). Those carrying costs are one form of a negative interest rate.
At the end of the day, someone is storing the money somewhere. And the root-level money storer in the economy is the Fed. If the Fed wants to charge you 0.5%/year to store your money, what are your alternatives? You could keep physical cash. But then you have to protect it. You could invest in other assets, like stocks/bonds/mortgages, but those have risks. If you can't find any investments you like, and you don't want to physically store the cash, you don't really have any other choice.