Underscores/underline were a hint on a manuscript to the typesetter that you wanted italic (https://en.wikipedia.org/wiki/Underscore). It later became a way to add emphasis (either italic or bold styling) on typewriters as there wasn't a practical way to have bold and italic versions of every character on a mechanical typewriter.
Most professionally typeset content (i.e., optimised for legibility through years of experience rather than just whatever your word processor will allow) will not contain underlined content (https://practicaltypography.com/underlining.html).
Underscores/underline were a hint on a manuscript to the typesetter that you wanted italic (https://en.wikipedia.org/wiki/Underscore). It later became a way to add emphasis (either italic or bold styling) on typewriters as there wasn't a practical way to have bold and italic versions of every character on a mechanical typewriter.
Most professionally typeset content (i.e., optimised for legibility through years of experience rather than just whatever your word processor will allow) will not contain underlined content (https://practicaltypography.com/underlining.html).