This modifier is usually used to style headers. It creates a line below the content in the cell. This can give your slides a professional look.
When to use it
You should use the Header underline modifier when you want to separate your headers from the rest of your content and when you want this look to be consistent and professional.
What can you change
You can update the color, width and thickness of the style. The width can either be set as a fraction of the cell width, or to Automatic which makes the line as wide as the content in the cell.
Update the style of your font. See the article about the Character style modifier for more information.