It makes sense that in a function which requires employment of locums to cover a full staff rota and which only has sufficient recruitment and retention to have 60% of full time posts filled, could reduce their overall expenditure this way if the locum cost is more than double the contracted-employee cost.
Full staff would be 10 people working 35 hrs per week (350 total hours) being paid £50,000pa each so total would be £500,000pa if only they could recruit and retain that many.
Actual staffing is 6 people working 35hrs per week being paid £50,000pa and 4 vacancies covered by an ever-shifting tide of locums which cost £120,000pa per vacancy to cover what with higher locum pay and additional admin costs so total for staffing is £780,000pa
If by reducing hours for a "full time" staff member to 28hrs while retaining pay they are able to recruit to a full department of 10 employees then they can get 280hrs per week of work for that £500,000 and can halve the number of locums they use to fill the remaining 70hr pw of unstaffed time so the total is only £740,000 - that's a £40,000 saving and worth doing.
But if the Locum cost is only £100,000 per vacancy rather than £120,000 the maths doesn't work out - 6x35hrs per week staff plus 4x vacancies covered with locums costs £700,000 and 10x 28hrs per week staff plus 2x remaining fte hrs covered by locums also costs £700,000
In departments where locums are not used, but there's just an understaffed team and an ever-growing case workload, there's no savings to be made there but if they have a similar vacancy rate, then 6 staff members struggling to provide a service with 210 staffed hours per week of effort would certainly be happier and less likely to burn out and quit if they were instead 10 staff members working 28hrs per week each and contributing a total of 280 staffed hours per week. But would retention and job satisfaction be just as good if the job was just given a 25% pay rise but number of vacancies cut, so that they have 8 full-time 35hr per week staff members giving that same 280 staffed hours per week between them?