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What's the point of using that when you have the preview tab which is to enable you see what you want to post and how it will appear on the forum, this way you can check for errors in spelling, grammar, quotes, image and so on. Notepad does not show these so I don't see any need going through that lengthy process for something the forum have a better option.
Although, Op didn't give a better explanation but I think why he brought this up is not all about previewing your post and making corrections, it more than that. The notepad comes very handy because there are times you may be typing a lengthy post using the forum's editor, there may be some network challenge at your end. The page would reload and you could lose everything you've typed.