Back in the '90s a firm could use a workforce consisting of self-employed people who were therefore technically subcontractors. This meant that the firm did not have to give holiday pay, sick pay, and I think it affected the insurance situation as well as the worker having to sort out his or her own tax and NI. Workers could also be told their services were no longer required without any notice as they were not classed as employees.
This practice was stopped by legislation, which I think said that if a worker worked more than so many hours a week for one contractor then that contractor had to become an employer.
If I have got any of this wrong I would be happy to be corrected.