Drafts are useful in cases where you are posting a long thread and don't have time, or simply don't feel like finishing it in one go. Of course, those are not the only cases, but you get the point. Drafts are simply used so you can save your thread and finish it later before losing what you have already written.
Creating Drafts
When creating a new thread, next to "Post Thread" and "Preview Thread" buttons, there is a button called "Save as Draft". Clicking that button, instead of posting a thread to be publicly visible, you will save it as your draft which is private and visible only to you. (See images below)
Editing Drafts
There are multiple ways of accessing your drafts, of which some are:
1. Options → User CP → Saved Drafts (click)
2. Options → Saved Drafts
"Options" menu is located on top of the site on the left side from your avatar picture.
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