documents vs mydocuments and publicdocuments What? Why? How? When?  

I'm wanting to learn some history and fill in some gaps in my general knowledge and experiences using Window over the years.
I would like to know all about what those Mydocuments, Mypictures, My[etc] folders are for. What are they? Why did they suddenly appear way back when Windows was young? What extensions of the concept are there besides Publicdocuments, etc.? Will new ones be created in future versions of Window 10?
Not knowing the answers, I have never looked into the issue of taking advantage of them to do anything. Early on, my interest in them all but disappeared when I tried to investigate them on my PC only to find that I had no permissions to access them. My conclusions were: 1) the system must need them for something, but I sure don't; and 2) it will be really interesting to learn what they are all about if their names say they are mine, but I have no permissions to them on my own system without making changes to the way the system created them from nowhere without my explicit "permission".
Now that I am a really old guy with a lot of time on my hands, and have never been aware of any problems caused by my ignorance of them, my curiosity about them is greater than ever.
This is all very low priority. I still don't know of why I may ever need to use them. Sharing other public folder files locally and on my LAN has never been an issue for me.
My apologies for such a long post for a simple question. I did lots of searches before posting, but with no success. If there is a better place where this thread should be moved, that's ok with me.
I'll be grateful for any info I can get to help me educate myself.
Thanks,
BadBernie
** I can use all the help I can get **

1) They are junction points and they are there for retrocompatibility.
2) You don't need to use them or worry about them. They work in a transparent way for the user.

BadBernie said:

I would like to know all about what those Mydocuments, Mypictures, My[etc] folders are for. What are they? Why did they suddenly appear way back when Windows was young?

They are all Junctions, pointers to actual folders, but with the names they were known as in previous versions of Windows. They are there for backward compatibility with older software that tries to save to the old default locations. For example, your 'Documents' folder is a real folder, but in Windows 95 and XP it had the name 'My Documents'. The Junction called 'My Documents' redirect older software to the Documents folder so that it will still be able to work normally. Your hidden Appdata folder is where apps save their working data, but it has variously been known in the past as 'Application Data' or 'Local Data'.

The current names (Documents, Photos, etc.) seem unlikely to be changed in future versions of Windows, so I don't expect any new Junctions to be required going forwards.

SysInternals said:

Directory symbolic links are known as NTFS junctions in Windows. Unfortunately, Windows comes with no tools for creating junctions—you have to purchase the Win2K Resource Kit, which comes with the linkd program for creating junctions. I therefore decided to write my own junction-creating tool: Junction. Junction not only allows you to create NTFS junctions, it allows you to see if files or directories are actually reparse points. Reparse points are the mechanism on which NTFS junctions are based...

https://docs.microsoft.com/en-us/sys...loads/junction


These are many other Junctions in your user folder, all with the same purpose - backward compatibility. Here are the Junctions in your user folder, recognisable by the L attribute which means it is a Reparse Point.



Note the dates. They were all created when the upgrade to 1903 was installed on this particular machine (the upgrade to 1909 was technically a minor update, not a new install, so the dates remained unchanged).