She was used in Her because she has a dry/monotone/lifeless form of diction that at the time seemed like a decent stand-in for an non-human AI.
IMDB is riddled with complaints about his vocal-style/diction/dead-pan on every one of her movies. Ghost World, Ghost in the Shell, Lost in Translation, Comic-Book-Movie-1-100 -- take a line from one movie and dub it across the character of another and most people would be fooled, that's impressive given the breadth of quality/style/age across the movies.
When she was first on the scene I thought it was bad acting, but then it continued -- now I tend to think that it's an effort to cultivate a character personality similar to Steven Wright or Tom Waits; the fact that she's now litigating towards protection of her character and likeness reinforces that fact for me.
It's unique to her though , that's for sure.
Do you have a source for this?