Pulling docker image by digest

Question

Why is it necessary to give both the name and the digest when pulling a Docker image?

docker pull ubuntu@sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2

Isn't passing the digest enough, or is the digest not unique enough in the context of the entire docker repository?

Consider the following example:

docker pull sha256:45b23dee08af5e43a7fea6c4cf9c25ccf269ee113168c19722f87876677c5cb2