Capacity at any time can be used to increase growth in flight numbers.
Here are just two examples:
There is capacity for European SEN arrivals from 0730 onwards on weekdays with departures 30 minutes later.
Also for middle of the day w-pattern arrivals/departures which would be perfectly suitable for any number of once daily services, such as the OLT DRS route.
If the only routes that airlines operated at LON airports were during the morning and evening peak periods there simply wouldn't be a European airline industry.