See: https://forum.mattermost.org/t/important-underline-tagging-markup-necessary/12054

(Details do not seem to be easily entered on this platform.)

Summary (see above link for examples and details):
Underline functionality is frequently needed in Mattermost to convey things more clearly. We have found many users frequently needing to use underlines to more clearly and visually succinct (to demarcate or differentiate text). For example, sometimes we discuss emails that are sent and many senders prefer to use “inline” replies wherein they will reply just following a question or previous remark. In these cases we desperately need an underline function.

Again, for illustration see above URL on the forum.