Notebooks is a nice and soft way to bring a programming mindset to non-programmers.

Your actions are repeatable and can be stored in a centralized repository.

There are probably some macro abilities in QGIS (it is an amazing tool), but this means moving to "script first" from "click first".

I can see it for programmers. Here you can use industry standard python libraries (shapely, geopandas etc.). Nobody really wants to learn PyQGIS (the python interface for qgis). So while qgis is much more full featured for "desktop" gis (designed to compete with esri arcgis) i can see the use case here for people who want to build their own extensions and port code from this to other python projects more easily.

If you're doing primarily analytics rather than making a map, I could see how JupyterGIS might be a better choice.

(but you can always just use proj/GDAL at the import/export layer...)

I see a few advantages. For my work in particular, I have to rely on creating desk study reports via exporting PDFs from QGIS - this depends on export DPI, page size etc. Following that I have to pull those plans into e.g. Word and it's a messy system.

A python notebook would be a nice way of generating reports of GIS data in an interactive way without being forced to use pages, PDFs, and embedded image files.