qtree is logically defined (walled) file-system sub-directory under the root volume. It has many advantages over a regular folder, such as you can enforce quota (restrict size for users), during backups, it is more handy b’cos it is a modular directory, hence ndmp backups becomes faster. Most important, administration/managing shares becomes very easy.
Whereas, if you just use regular folder inside the flexvol, you cannot enforce quota/size restriction for users. During backup (especially ndmp), it can be daunting as the size increases. Plus, when you share it out, you will see all other shares which are under the root volume, instead of the just ‘qtree’ share that you shared out.