Struggling to understand why leaf nodes don't need wholes?

In the docs (section on schema, subsection Serialization and Parsing) it says

Leaf nodes don’t need a hole in their DOM representation, since they don’t have content.

I can’t clearly understand why it is so. Can someone please explain and provide an example where leaf nodes don’t have content?

They do not have content, so it is not necessary to specify where the content is rendered.