Underlining it is an emphasis.
It tells the audience the importance of the underlined word.
It is like pinning a word or making a word look different from others.
Most of these things are overlooked by some reader but it is really important.
Underlined word has a deeper meaning or an instruction attached.
I remember having to deal with the underlined word in English language when I was in highschool.
So, you don't underline it casually, it has to function in the context.
Underline it when it necessary.
Make the emphasis loud and clear .
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