Libraries are great. They were intended to be a handy tool for casual Windows users, but unfortunately did not get the UX clear/simple enough for casual users so it got relegated to the Power User tools most people will ignore (and de-emphasized in Windows 10 to the point where it's mostly only Power Users left that will bother to find Libraries and activate it).
The attempted explanation from the article.
> Windows 7 introduced this concept - Libraries, meta-spaces that grouped together multiple actual folders on the disk to create what database developers call a view. The Pictures location is not just a folder, it's potentially several folders. Don't worry about it, Windows will figure this out for you and simply present what you wish to see. An interesting idea I don't believe anyone wanted, or that in fact really worked at all.
Libraries work pretty great. There's still a few issues with them, mostly in specific applications that use older common dialogs for whatever compatibility reasons, but they do what they are designed to do: a Library is simply a virtual folder that aggregates other folders. The article's comparison is to a database view versus a table and that's an accurate analogy.
Any folder can be in a given library with one big caveat (and it is possibly the biggest that I've seen often trips casual users) so long as it is actively indexed by the Windows Search services (which is why it especially breaks the casual users that are the sort of mid-level almost power users that heard somewhere turning off Windows Search services was a "performance boost"). (Because it is Search services that provide the "view", and is the database powering the virtual folder.)
The default Libraries are based on the old school My breakdowns: My Pictures, My Documents, My Music, My Videos. But you can create your own Library of any combination of folders of interest to you.
One place it is very useful is for instance my Documents library. I can have one view that shows Documents I've got stored locally on the device only, stuff I've got in Dropbox, stuff I've in OneDrive, stuff I've got in iCloud, stuff I've got in Resilio Sync shares. It doesn't matter to me "where" I've got that particular file stored, I can search/browse the Library and access/filter/group all of the files in one place.
I can even set the default save location to whichever cloud (or otherwise) service I most trust at the moment, so if I just save a new file to my Documents library, it'll save to the Resilio Sync folder I chose this week. Last week it was a OneDrive folder, and maybe next week my needs will change again I will be back to the local-only folder. No matter which "where" it is currently pointing to, I can just select the Library in the File > Save dialog, save, and the Library will handle remembering which folder I wanted to save to this week.
OneDrive today offers the option to entirely replace the local-only Documents folder, and if that's the only provider I used that might be a good option. Libraries fit really well in this world where I have multiple providers for different needs and with different storage capabilities.
On my work computer one of those Documents folders might be a file share (so long as it is a relatively recent file server with up-to-date Offline and Search Indexing capabilities).
The same expands out to other roles like Pictures/Videos/Music. (I have multiple cloud providers for my music. I've got OneDrive and iCloud with different sets of photos. Etc.)
The idea is useful for other things. For instance, I sometimes have multiple folders for things like Git Repos, splitting them across drives due to file size for instance, and having a Repos library that shows me every Repo I'm working on no matter which drive it is physically stored on can be very useful.
You can also do some of that with NT Junction Points / Reparse Points, but that's even more extremely a Power User-only approach (and sometimes not recommended simply because of how many footguns are lying around there). Doing it as a virtual folder that's really just a Search View is all I really need in most cases.
The biggest trade-off is mostly that Libraries don't exist in the Command Line world, I can't CD to a Library in CMD. (If there were the right sort of Provider I should be able to CD to a Library directly in PowerShell, though sadly a quick search doesn't turn up an out of the box PowerShell provider for them; a shame.) But I can still open the Library in Explorer and select any subfolder in the Library and File > Open in PowerShell (Alt+F,R) it, and Explorer will do the work of figuring out which storage place it is in (its full path) for me.