I was using i3/sway for years previously as well (and some awesome, qtile before that). The big difference is window size.
Generally I believe most people like to order their workspacesroughly by topic, e.g. all work related Windows on one, browser or on another, some also do all terminals on one... Now with sway/i3 I often found myself in the situation where I was e.g. on the "browser" desktop and you read something you quickly want to try in, e.g. ipython, or you are working on a latex document and want to briefly open a PDF. In i3 that would reduce the size of your original window, so you end up switching to a workspace (or you manually switch to tabbed tiling) for me the mental overhead was significantly higher and I was ending up creating more and more workspaces just to hold temporary terminals.
This is actually related to why I switched to i3 in the first place, I just felt vertical tiling is the only tiling that makes sense in 95% of the cases and that just worked best in i3. But that comes at the cost that you are limited to only 3-4 tiles per workspace (depending on screensize) now in niri I have infinite theoretically. Which means I spend less mental overhead when I want to open another window (which is really thebgoal of tiling wms in my opinion, reduce thinking spend on window management)
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.