Often times, there may be 40-60-80 players on, but still not have that much cases/issues flowing in. Without the staff ratio, staff can adapt to the amount of issues/cases coming in. Similarly, if there is only 40 players on but a lot of cases 2 staff can't handle, others can hop on and help.
It can also sometimes give staff more incentive to get on staff without having to be constantly reminded/bugged about the ratio.