Basically the same reason you're using mosaic + the ability to change what exactly is being done. I use it to map 'hyper'+key (where hyper is the equivalent of ctrl+cmd+shift+option and is assigned to control, capslock is assigned to the original control functionality) to move windows between screens and on their own grid. Effectively I just didn't pay for mosaic-like functionality. I have better touch tool, but I never liked having to drag to snap and had hammerspoon implemented already.

My use case is to split the screen in two, editor on one side and terminal on the other. I specifically want the terminal to be 80 characters wide (inside that window I use tmux), and the editor to take up the rest of the space.

I've tried most of the simpler, GUI-based window managers for macOS but I haven't found one that let's me set window sizes explicitly in pixels yet. With hammerspoon I figure out the width of the terminal app I want, and adjust the windows accordingly.