They didn't work that way at Asda (maybe because it isn't just shelf stacking, which is only part of the Shop Floor Assistant's job).
But the point is that they are not jobs, they are work experience, which may lead to a job, and will also show potential employers that the applicant has the ability to turn up and do a job.
I (and I assume most employers) would not go out and take people on for 4 to 8 weeks on spec, on full pay, for the possibility that some of them might be employable - ymmv.
Just found a job ad for Work Placement at
Asda