Using One Drive to automatically back up My Documents folder

This solution would require you be online whenever accessing your documents... that's not an answer to the question posed.

Very helpful.  However, I must further clarify Step 2.  You need to right-click on the Documents folder on your "PC" and choose properties.  Then the "Location" tab will appear.  Then move your documents to One Drive.

Hey Mike, I'll try to hit your points best I can.

In general, why you have both is actually why the "Computer" folder is renamed to "This PC".  They just want customers to be able to tell, immediately and easily, whether a file they're saving is being saved in OneDrive or locally on their own PC.  Now for way too much detail you'll hate me for.

Should I just use onedrive folders and delete contents of same name folders under "This PC" ( I know I can't delete the actual "documents" folder).

You can't delete those shortcuts you see in the "This PC" folders.  I say those are just shortcuts, because they truly are. The files are stored under your user account, at a place like C:\Users\Mike\Documents.  You can hide this shortcuts (mostly) by right-clicking a blank space, choosing Group By/Type, and then shrinking that group up to nothing.

Where these shortcuts point are defined by a variable. Since these are the default folder provided by the shell (the operating system interface), they're called User Shell Folders. If you right-click one, you can set where the folder is really stored.  This is useful if you want to force your Videos folder to be stored on another larger drive, yet still be easily accessible from various points in the interface.  IT administrators also use this redirection to store people's files on a central file server, but allowing them to work on their PC as if they were local files, oblivious or at least agnostic to where the files really lived. Replacing an employee's PC is easy because none of their files are actually there.

This is all on a PC is there any point in having the one drive folder available offline.

I certainly think so.  If you have the space to do it, do it.  There's a few reasons I say so. One: redundancy. My backup strategy I've been taught is a 3-2-1 strategy.  It means you have 3 copies of your data, stored in 2 different forms, and 1 off-site.  It sounds complicated, but using OneDrive makes it easy.

I store my files on my PC, available offline, inside the C:\Users\Shawn\OneDrive folder (and there's a handful of folders inside of course).  That's my local copy.

The OneDrive app icon running down by my clock makes sure that a copy of these are duplicated basically in real-time online. That's my off-site copy.  In addition to backing up the files for me, it means I can also get to them anytime from a web browser, even if I leave my laptop at home (or my laptop burns in a fire, destroying my local copy).

I own an simple external USB hard drive, and I've setup the Windows "File History" feature to make regular backups to it.  These backups are what they call "versioned", and means that it keeps more than just a copy of everything you have.  It makes a new copy every time a file changes.  So it only has 1 extra copy of, say, a family photo from years ago.  But a spreadsheet I use every week to track inventory, when a backup runs and it sees that file has changed since the last backup, it keeps both copies.  And again the next time.  With this you can browse back in time and open up the file as it existed yesterday, last week, or last year, or any point in between.  The only limit is how much space on the drive, and it drops the older versions when space is needed.  (Recent versions of changed files are also available on OneDrive.com and works similarly but totally independently and doesn't rely on the computer's version backups).

It's important to note that the Windows File History backup can only run if the extra hard drive is connected (if not it will keep trying and eventually ask you to please let it do a backup).  Also, it only backs up files that are available offline.  If OneDrive is only using the "online only files" which are essentially just shortcuts, then the File History has nothing to actually backup, and now you've lost 2 of your 3 backups in the strategy (the local copy and the extra hard drive copy).  And while I think OneDrive.com is reliable, I don't trust anything enough to risk losing all my files over a hacked password or giant service failure (it is a free service, after all).

If you want, you can use that redirection I mentioned to set the location of your Shell Folders to truly store their contents inside your OneDrive folder.  If you want anything you save in This PC > My Documents to actually store inside C:\Users\Mike\OneDrive\documents, then that's your trick. I don't do it myself because the local MyDocs folder is only populated by default junk that various programs dump in that I either don't want or would never need outside the context of this specific computer (like default preset settings files for my video creation software). I never visit this folder and anything I care about is actually stored in OneDrive.

This is the very first time I've tried to use a forum, it's absolutely astonishing the what I see as very simple questions can't be answered by the mighty MS.

Many thanks in advance

PS. Libraries (de-emphasized in Windows 8.1 onward) are different and basically saved search view. Right-click a library and go to properties, and you'll see all the locations it searches. Normally this is just your own folders, but if you share a computer or network you can do what I've done.  My Video library is a view that shows me all the movie files stored in my extra hard drive, plus my other computer's movies folder, and my roommate's video folders.  So in one library view, I can see all the video files I have access to.