Vim + tmux with custom bindings makes moving between vim and a terminal seamless. The same keystrokes for moving between panes in vim can be used to move to tmux panes. Also, as a vim user I loathe to reach for the mouse, so having quick and easy keybindings for launching and arranging tmux panes is great and preferred to the imperfect and slow arrangement using a mouse. Lastly, I can have tabs and window panes that are named and in consistent locations and persisted across sessions, so I can immediately jump around a dozen projects without much cognitive load. Try having 15 terminals open and remembering which is which and you'll find there is a high context switch cost, so you constantly have to close and open terminals. This isn't the case with tmux.