Maggie O’Farrell explains:
I had a really bad stammer in my childhood and adolescence, and that imbues you with two things. First, a hyper-sensitivity to grammar, because a stammerer will have problematic sounds, impossible verbal stumbling blocks. To get around these pronunciation traps, you’re editing and rewriting sentences in your head before you say them. Second, writing is just such a joy when you have a problem with speaking. It’s so astonishing to watch language coming out of your pen without any hesitation or dysfluency. … Stammerers become skilled at sentence construction and synonyms: we have to be. Faced with a problem word, we need to have instant access to eight others we could use instead—ones we could say without stumbling.
The Dish 