What can I use them for? Some concrete examples. I feel like I might be missing out on something.
Not sure how much help this will be but I'll share my experience anyway. A usage that has worked well for me is as follows -
I been using Blender in place of an architectural CAD package to design a small summerhouse. I have a main scene in which all the elements are drawn (slab, landscape, sole plate, studs etc.) and these are all placed into collections in that scene to help organise them and manage visibility.
If I now want a drawing of a particular subset of these elements, I can now create a scene for it and link in the elements I want to show on that particular scene. The linkage can be by collections but it is also possible to link in individual elements from the master scene. I can now create a new collection in that scene for the elements that are to be present only in that scene; into this local collection I place a camera and possibly lights and adjust the scene settings to use this camera for renders. I also set the camera to be isometric to get a plan view, use freestyle to render the line edges and add dimension text to the scene local collection and use material override to render all objects in a plain grey material. I can now render this out and effectively get a plan view line drawing.
This now means that I can return to the main scene and continue to add elements to it and, as long as I'm carful about keeping things in the right collections, these new elements will also be visible in the other drawing scene (if that scene links that collection). The main scene still has its own camera and renders from this scene will be in perspective and with materials rendered.
This workflow loosely recreates the feature in CAD packages where you can have a number of drawings with different views and subsets of the main model. In my workflow, each scene is a drawing.
This is possibly not what the scenes feature was primarily designed for but I do find it it useful in this case.