Because the alternatives is basically to store reserves at the central bank or to store them in cash, and storing in cash is costly. The reason for this is tha

Because the alternatives is basically to store reserves at the central bank or to store them in cash, and storing in cash is costly. The reason for this is that banks within a banking regulation/currency area is a closed system - reserves are created when a loan is made, and cannot be removed from the system until that debt is repaid. A bank can store it in actual cash or in reserves in another bank, but that would simply move the problem to that bank. They need to be able to store their reserves, and they end up doing that at the central bank.