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.
You're right and I have experienced this several times concerning my network issue, I never thought about notepad when I experienced this and it could be of good help but I tried changing my browser and it worked, using a good browser can help solve this problem of network leading to reloading of a new page, instead of a new page the browser will take me back to my writing.