It allows leaving placeholder in the template to be filled with any HTML from inside the tag when used.
For example, these 2 initial paragraphs will be place where the "intro" named slot is in the template all because these paragraphs have the slot attribute with value of "intro" to match.
This one will be grabbed by the un-named slot since it does not also have a specified slot.
This paragraph will be ignored since the slot value does not match any existing one in the template.