Even when there are no reserve requirements, banks often as a matter of prudent management hold reserves in case of unexpected events, such as unusually large net withdrawals by customers (such as before Christmas) or bank runs.
Traditionally, central banks do not pay interest on reserve balances, but such schemes have become increasingly common in the 21st century.
The amount of money needed to be at call varies because of a number of factors.
For example, there is a higher demand at Christmas time when commercial activity is highest.
There may also be sudden, unexpected surges in demand for cash by individuals during economic panics, which may result in a "run on the bank" as individuals seek to withdraw money from bank accounts.