Lots of tasks involve two different apps (googling a bug while looking at the error, reading a spec while working on the code, being on a call while showing off a document etc). I'm almost always happier with two full monitors, but I use tiling when I'm stuck on a single laptop screen for example. I rarely want more than two things at once unless I'm in Emacs, and I sometimes get the sense that a lot of tiling window manager and tmux use is just people not knowing how to use their editor's built in window management and multi-process support. Obviously everyone's free to build their own environment however they like, I don't know why anyone would accept being stuck on a tiny screen for long periods though!