Is it redundant in a Dockfile to run USER root since you're already root?

Yes, it's redundant, but there's almost no downside to leaving this redundancy in. This may have been done to develop against other images, or to support uses that may swap out the base image. It could be done to prevent future issues if the upstream image changes it's behavior. Or they may just want to be explicit so it's clear this container needs to run commands as root.

If an image was generated from a source that changed root to a user, you may not have access to all resources inside it. However, if you load the image:

FROM xxxxxx/xxxxxx:latest
USER root

That will give you root access to the images resources. I just used that after being refused access to change /etc/apt/sources.list in an existing image that was not mine. It worked fine and let me change the sources.list

if you are already root, then it's redundant to use it.

As @BMitch also points out, you can use USER root to ensure you are not going to break things if the parent image changes the user in upcoming versions, among other things.

It actually depends on the image. In some images, such as grafana/grafana, the default user is not root and there is no sudo. Thus you must use USER root for any privileged task (e.g., updating and installing apps via apt).