what is a .fuse_hidden file and why do they exist?

You can safely ignore .fuse_hiddenXXXX files. It means a file was deleted but there is at least one software which is still using it, so it can't be removed permanently.

It will be done automatically when the relevant software stops using the file or exists. Such files are always gone after umount/reboot. This is how Linux and any Unix works but only FUSE exposes these files to the user.

These are likely files that were meant to be deleted but are somehow still in use.^UBF