Back in the olden days of the last century, I was recruited into the civil service in that manner.
A campaign would be run by a regional personnel branch, you apply and get interviewed for a grade (CA Clerical Assistant back then, which is now E2), then assuming that you passed the process then your name went on a list
When a branch had a vacancy they went to personnel, the list was consulted and you
were offered a job on 6 month probation.
This process had the direct cost of employing a personnel branch in each region and running campaigns as often as needed to maintain a big enough list to increase the chances that someone would still be willing to still be sat waiting for a job 6 months later
(If you have no vacancy’s to recruit then you still had the cost of a branch full of people)
Regional branches became more centralised and then eventually become just the recruiting policy, advice office etc
Nowadays a manager with a vacancy runs their own recruiting with the central department providing the advice, publishing the advert and ultimately co-ordinating the job offer and onboarding
The manager is the default interviewer and requires an independant as 2nd interviewer.
Rather than a branch being paid whether there’s a vacancy or not, I was the independant recently
For my time it took a half day reviewing applications, about an hour of both of us comparing our scores, then two half days of interview.
The line manager had the time to initiate, to look for an independant and to type/upload the scorings from sift & interviews.
With the two of us added together it might have been a week of work across her campaign.