I still don't quite understand the value of tmux, at least on a modern machine. If I need another terminal, I just pop open another terminal window and tile or tab it. If I want to keep a set of terminals organized, I use workspaces (at the OS level). I can obviously see its value if there is no window manager or desktop environment on the system I'm using, but that's rare these days–if I'm working on a system with n…
On Vim there's a plugin called Vimux which allows me to control other pane from Vim. For example, I type `l` and it runs the last command on the next pane, which usually is a command to test the code I'm working on.
Sure, you can do everything in a WM or DE, but I use Linux and macOS and it's nice to have the same configuration, keybindings and environment everywhere. I also find tmux more customizable than Gnome Terminal and easier to setup than st or urxvt.