For non-code writing such as academic papers, notes and things written in markdown. All of which feature a mix of code, math, and plenty of prose. I don't love writing prose in monospace, and I also don't love using multiple tools for one task, and I also don't love using the pointer on the laptop (as oppposed to navigation keys in vim-like environments). With Atom, I can kill all of these birds with one editor.
Write 'normal' text for example, also if you're including CJK-strings in your app it is kind of nice if they are not rendered as teensy or on top of each other.