in electricity networks, generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power systems are designed so that, under normal conditions, the operating reserve is always at least the capacity of the largest generator plus a fraction of the peak load.
It is better to have too much energy available than not enough.
If the margin is too big you are wasting money
Any discussion about replacing non-RE must be considerate of operating reserve margin. The larger the renewable generation, the more operating reserves will be needed.