Thank you for the explanation! Between your explanation and a Youtube video I found of someone using it, I think have a grip on the "why" now. Interestingly, it seems like I might use i3 a little differently than you did. A single working context for me can span many desktops, and I just work in a way that keeps my number of concurrent working contexts low. I only ever have 1-2 programs on any given desktop, and even 2 is unusual. I keep every window in tabbed mode. When I need a scratch "thing" (nautilus, terminal, localc, whatever) it opens as a new tab. If I need it side-by-side with that desktop's primary window, I pop it out using mod4+shift+left/right. This accomplishes a similar thing to that Niri is getting you, just with different ergonomics. It probably helps that I have good habits around closing unused tabs / programs.
Yes it seems like you're using i3 quite differently. I agree that if you good discipline about opening and closing windows in the right workspace/tile i3 can give you a more structured layout. I just found for myself even if I tried I could not keep the discipline up (I doesn't help that I often work on several things at the same time).
I think that's the beauty of tiling WMs (and I consider scrolling WMs a subset), you can really adjust them to suit your work flow even if work flows might be very different. In contrast stacking WMs seem to be more a lowest common denominator type thing. They work with every workflow, but suboptimal.
I think my i3 "style" came from using a 12-inch thinkpad for many years. The small screen size and low resolution force you to work in certain ways. And if I had left 100 Firefox tabs open on that machine's second-gen mobile i5, I think it would have melted straight through the desk.