What are folders within sites/default/files for?
They're created by different parts of the system, when they're required, for pretty much anything at all that a module needs to store file-wise.

It would be pointless to try to list them all, as core/contrib/custom modules can place files/folders there arbitrarily; Drupal doesn't place any restrictions on what can/can't be stored there, it's up to each bit of code as to how and when it uses that folder to read/write.