i3wm is a window manager on Linux that is just a bit different. It's a tiling window manager, everything is nicely organized.
i3wm is for the Linux desktop only (as far as I know). If you have few screen space, like a laptop screen, why not use all the space you have available?
i3wm helps when getting things done, when you're working on something you don't want to be distracted.
Distraction free work is one of the biggest productivity boosts you can have. That means no notifications, dragging windows and all those things.
All those dinky-toy decorations and effects aren't really helping when you want to get work done as quickly as possible.
Now i3wm is not for everyone, it takes quite some time to get used to and is not suitable for all use-cases.
But when coding or doing things on the terminal, it's just perfect in my opinion. In general I like it for coding work.
For other purposes (like maintaining servers), you may only want the terminal as that saves cost and easier to maintain.
If you are doing video editing with openshot orsome other program, it may not be the best tool for you. For this kind of use cases, kde may be better suited.
There are other tiling window managers, on top of my head, bpwm and awesome, but I'm used to i3.