This question is lifted from author Sahar Sabati's informative Ask an Author series, which ran from 2015-2016 and included responses and reflections from English-speaking authors from around the globe. Since I'm often asked these questions by readers, I have republished my answers here with Sahar's kind permission.
Being an editor certainly helps with my writing ‒ you can’t help but be on track with genre trends, and writing styles. Seeing other people’s errors, and how certain techniques can be used to remove those errors, thereby tightening and strengthening the narrative, ultimately helps me to improve my own writing. It’s comforting to know I’m not alone struggling with a sagging plot, a lacklustre character, or a dwindling-down-to-nothing ending. But when it comes to reading, being an editor can spoil your enjoyment of a story if the text is muddied with spelling and grammar issues, plot holes, and consistency issues. We can’t help but see them. They jump out at us more than the mole on an ugly stepsister’s chin. Some self-published writers will tell you their readers are more concerned with the story, than things like typography. They’ll tell you the odd error doesn’t really matter, but it does. I have a writer colleague ‒ not one of the Ask an Author stable ‒ who writes bestselling historical romances. An indie writer, she spins a fantastic story, full of period intrigue and rippling muscles and romantic angst. But almost every speech in her books are incorrectly punctuated – resulting in up to six or seven errors per page. They jump out at me, distracting me so much that I can't enjoy the story. I’ve alerted her to the problem, but it seems neither she nor her editor understand standard punctuation rules. Sadly, I can’t read her work anymore and she has lost a valuable reader and reviewer. I wonder how many other loyal readers have drifted away for the same reason.