Well, I can’t speak generally to you being a blockhead or not.
The advice is because you have to get a GUID from someplace, and then keep track of them all. Using the computer account GUID is a shortcut, because AD is going to keep track of that for you. So you won’t have to have another list of “Computer=GUID” anywhere. But it does assume you’re in an AD environment, and DSC is otherwise not dependent on AD. This certainly isn’t the only way to do things - but it’s one way, and some people have been happy with it.
But this kind of all goes away in v5 anyway. Configurations don’t use GUIDs anymore, they use a human-readable name, so you have a lot more flexibility and probably an easier time keeping track.