What is the different between LoadSceneMode.Additive and LoadSceneMode.Single? When should I use one, over the other? I read the documentation, but I can't find any good examples of use for a definitive situation.

The document is actually pretty self explanatory, though the documentation for the new implementation is not as helpful as the original documentation. Let's take a look:

enumeration

## Variables

This tells us everything we need to know about the two; If you use LoadSceneMode.Single, you close the current scene, and load the new scene. If you use LoadSceneMode.Additive, you leave the current scene open, and load the contents of the additional scene into the current scene.
If you were to have a game with distinct levels, where a player directly moves from one level into another, you would probably use LoadSceneMode.Single. Think of it like a level of "Super Mario Bros." or "Rayman". Once you move in to the next level, you do not want to use the contents of the previous level. You might be able to move from level one to level two, and you might even be able to move back to level one from level two; but you would not be able to be in both level one and level two, at the same time.