The CSCS card came to be in the wake of the New Roads and Streetworks Act of 1991. That piece of legislation addressed the fact that many people working on roadways in the UK had very little training and were prone to causing accidents. The NRSWA required people working on UK roadways to receive the training and certification they lacked and set up a mechanism for proving they received that certification: the CSCS card.
Today if you want to work on roadways in the UK you will need a CSCS card that indicates your proficiencies and that you have passed the CITB HS&E test.