Unfortunately it is not really about the 'contractors' in an IT sense. IR35 was introduced into law in 2000 by Gordon Brown to counter 'tax avoidance' by the use of so-called "personal service companies", specifically highly paid BBC presenters who were advised by the BBC to use PSC. It is alleged that this was to help the BBC avoid paying the Employer NIC contribution on their earnings.
By the way, there is still no definition in HMRC of what a PSC actually is - one of the many problems of this incredibly badly thought out 'tax and grab' legislation.