I think underline is used to make emphasis.
It points to important facts.
Maybe to call your attention to the fine details.
It is really a good way of communicating an idea without making a big deal out of it.
I make use of the underline when I decide not to use capital letters or bold function.
Some folks may see underline in a different way, and also use it differently but I see it this way.
Sometimes the headings are underlined, sometimes words in the body part are underlined, you have to check what it means contextually.
An underline is a good function, as it redirects our focus.
So, keep underlining.
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