When you have two parents with the same name, or whose payment information (legal name, email) might not match what's on file, it is extremely helpful if they are able to include their unique identifier: the user ID.
This would also be useful for students.
We frequently use User ID when exporting to other systems (ex. school pictures), and students need to provide this ID when using the other systems. It would be helpful if they can pull this ID on their own.