I do understand the concept of layers being mostly physical. But that just means that there are limitations for what you can do with the layers or not. And it is not completely true that you can not something like that. For example take footprints. You can think of them as collection of layers that you map onto a physical stackup. Only their behavior is so deeply engraved into the KiCad (or any other EDA) that you do not think about them like that.
And yeah, I can make a copy of footprints on the user layer or export it to svg and re-import or any other way I can think of. But that is this one example and even here it adds an extra work.
But I can find more examples that could be solved easily and more intuitively by using layers. For example some time ago I’ve been reading here about board variants: How to manage board variants (different components and footprints)? - #6 by poco
You can see that most of the solutions are a really awkward and feels more like a work-around-work-around than a proper solution. And they do not offer solution to variant tracks. Even one of the link on gitlab links to a video how it is done in altium: The variant is a SMD or THT LEDs. And the solution is to make tracks for both of them and position them carefully so they do not conflict… How much easier it would be to have one layer for SMD and one for THT?
Or imagine visual diff directly in KiCad. You would just second PCB into separate layers and you would immediately see the differences.