Because I don’t like things to be behind other things. And I feel like moving windows around is a waste of effort and time, and also requires using the mouse where I wouldn’t normally have to. Tiling windows and using workspaces to organize my work/play/attention works very well for me and helps keep my focus where I want it.
Also sway in particular, but other tiling window managers too, have better output management than standard DEs. If I’m on output 3 workspace 12 and I want to do something new, any new window I open stays on output 3 workspace 12. I have a lot of displays and not being surprised about where windows open is extremely helpful.