This is a useful way to save overriding and overlaying textures on existing prefabs over and over. The main advantage of turning an object into a prefab is that it makes it easier to update and change multiple objects at once that are the same. Also if these objects inherit shaders used elsewhere you can make modifications to textures easily to all of the objects that exist as ‘that’ prefab. Now that may sound overwhelming and gibberish but it will make sense why this saves time the more you use Unity.
For example, if you download a tree from the asset store but you wanted to swap the materials out, rather than place and drag textures over and over you could unpack the original prefab, modify it and turn it back into a prefab which would allow you to load the prefab into the terrain editor or simply place down multiple trees that are the same.
Part 2: See how quickly changing custom objects can be once they are their own prefab #
As mentioned above once an object has been converted to a prefab it is easier to update and make changes to all of them.