What is the Pictures folder?

Pictures Folder is just a folder like any other.

For example, Documents is a folder that lots of apps like to use to store their pdfs or docs, or whatever. They could be stored other places, but Documents is a handy place, so many apps are designed to use it.

Like that, Pictures is the default location for data used by Photos, Photobooth, Adobe Lightroom, and a bunch of other apps that process images. I also include some folders of my own pictures that I want to access independently of those other apps, like the RAW files of my Nikon pictures, and scans I have done of old family pictures. Some of those, like the scans, are things I want to mess with before I import them into Photos, and some have picturesI have Exported from Photos for some specific repeated use.

You are welcome to trash anything you don't want. Apps that store their stuff there will recreate their folders if they need them but, of course, the previous pictures will be gone. If you delete the Photos Library, for instance, you'll lose all your pictures, but the next time you run Photos a new empty Library will automatically appear waiting for new pictures. Since I have a fear that I would later find that I really wanted some of those things, I'd move them to a flash drive or an external backup before dumping them.