I often need markdown, usually in GitHub issues when I am corrected in lower comment or somewhere, I strikethrough the wrong content and then write the correct one.
It’s also useful when situations change like today at one Discourse I first talked about ~Manjaro~~ and then moved to talking about Antergos and strikethrough would have been helpful.
Googling I learned that <s> and <del> exist, but I am already used to GitHub flavoured markdown and probably many other people too.