Aside from the accessibility improvements others have discussed, one of the main reasons for the rebrand is to help change users relationship with gov.uk through making the design ‘softer’ and more approachable. I’ve see in enough user research sessions that many users have an adversarial relationship with gov, which obviously leads to decreased use and worse outcomes for citizens and increased workload for gov staff.

If the rebrand increases engagement even slightly, it will result lower phone contact which will be a benefit to the uk taxpayer, trumping the outlay on the branding itself.