"keep file organisation" checkbox

You mean "Keep folder organization"? This checkmark in the Import panel will import the photos into albums, wehen you import photos from several folders at once. Photos will create an album for each folder you import, and if the folders are nested, it will create containing folders for the albums.

This way the folder structure on your disk will be reflected by the albums and folders in your Photos Library.

It is a new feature in Photos 3.0 on High Sierra.

Photos is getting more versatile with every system upgrade. I hope, Apple will keep it up 🙂