If you only use the terminal, you don't need a tiling WM because tmux and others offer similar functionality out of the box.
Tiling WMs are useful in the opposite scenario, when you use many different apps and need a unified way to manage them all.
I just can't imagine working in an environment where you have, say, 4 apps occupying 4 parts of your screen continuously.
Splitting screen in two (terminal and browser) is the common scenario for me but you don't need i3 for this.
For example, maybe I want to keep an eye on several terminal commands at the same time. With i3 I just press the terminal shortcut n times and I have n new terminal windows sharing the space. If one of the commands fails with a long error message I press another shortcut and the terminal windows are now tabbed and full height, with the IDE still visible on the side.
Or maybe I want a small browser window or Slack window in a corner to keep an eye on a meeting or discussion while I work.
Whatever uncommon layout best serves my needs right now, i3 can get it done in seconds.
You never really need a tiling WM, though. i3 just solves my problems really well, so well in fact that I actively enjoy using it. Some people feel the same way about vim, and you never really need vim either.