In reply to aln:
Three different (but not mutually exclusive) reasons:
1. Because it was part of Dominic Cummings's deal with Johnson. He's a huge admirer of the USA's ARPA agency in the 60's, and he claims he insisted on creating something like it as a condition for joining the government. He gave it enough momentum in government that, even after he left, the process of establishing it kept on rolling on.
2. Because the government wants to create a science funding agency that is less burdened by the bureaucracy and political interference that it has itself imposed on the main funding body, UKRI.
3. Because in principle it's worth trying out different ways of funding science, because no-one knows how to do it perfectly. This body aims to give much more freedom to a programme manager to commission research that advances their own vision, rather than waiting to see what proposals come in from the scientific community. The example from ARPA is JCR Licklider, whose vision of networked computing helped get the internet going.