"I seem to remember, in discussion with an acquaintance who was in social work, that in Scotland, the revised SW registration process required all addresses from birth or something - and all employment history, without exception. He may have been exaggerating a tad"
He wasn't exaggerating, that is standard for SW's. You cannot leave any time period blank, even if you put "caring responsibilities".
It will be for background checks, not everyone's info will be checked, but occasionally, for them to be considered a reputable agency, they will be expected to check randomly.
They should also have details of clients on file, if they become involved in crime, the police will cross reference, other agencies can use this information, also.
Likewise if a body turns up, and an agency payslip was found (or something else), the info could be of help.
It is easy to be involved in criminal activity and pick up work, this just safeguards us all.
The Yorkshire Ripper would if been caught quicker if this sort if info had of been held. The Police do check agency if they think a serial criminal is picking up work, whilst moving about committing rapes etc. to put it simply.
It makes more sense when vulnerable immigrants etc are looking for work, but go missing etc, someone somewhere should know who they are.
This cuts down on people completely disappearing and nothing being known, the UK is a relatively safe place to live because of it, or at least, it is difficult for murders/kidnaps to go unnoticed.