Discord is after Slack's market share and offering threads makes sense in many contexts where the discussion isn't casual.
And its just annoying to see all the duplicated text. If you have several people in a discussion or there's two threads of conversation going on I sort of get the idea that you want to respond to a specific comment -- but there are ways to use the English language to indicate clearly what you're responding to. Even if its just "re: whatever, ..."