I thought Swanwick was supposed to make holding a thing of the past?
It pretty much has I think. Can't recall seeing any en route holding for years now, except in emergency situations.
Now if you meant Terminal Control operations (which is done from West Drayton) ......
then holding provides a reservoir of aircraft which helps provide an optimised maximum arrival rate since ATC can constantly keep the stream of inbound aircraft on the approach topped up, as well as managing arrival streams to provide the maximum number of aircraft per hour taking in to account vortex wake separations or physical constraints such as bad weather.
In addition, there is a maximum limit to the number of aircraft per hour which can occupy the tarmac (both runways and stands). Look at the airline timetables and you will find that the airlines take little account of this when writing their schedules. There may be around 20 or 30 aircraft planned to depart or arrive at the same time which obviously is a physical impossibility in reality. So someone somewhere will either pick up a delay on the ground or in the air.
One of our Terminal ATCOs (the ATC speciality and not an indication of their health !!) can probably elaborate further but that's the basics.