A static or roller sun shade on the rear window reduces direct sun on a rear-facing baby, which matters more than it sounds — babies can’t reposition themselves away from glare or heat the way an adult passenger can.
A baby car mirror, positioned so you can see a rear-facing baby’s face from the driver’s seat via the rear-view mirror, solves the genuine anxiety of not being able to check on your baby while driving without turning around.
Neither item is a legal requirement, but both address a real, common source of parental stress on longer journeys for a relatively small cost.