#1. Spelling. Let's be honest. Nobody takes you seriously if your writing or poetry is full of spelling mistakes. Spell Check can only catch so many mistakes and typos will often sneak into a finished story or poem if you don't have an editor.
#2. Grammar. Again. Same problem. People make mistakes and readers (and publishers!) will judge you based on those mistakes.
#3. Story Consistency. The villain's hair is black in one chapter and blonde in another? Whoops?! Editors will notice this inconsistency and point it out so the writer can fix the problem. Did the hero injure their left leg or right leg back in chapter four? Hmm. Better fix that because they really shouldn't be sprinting across the parking lot in chapter six.
#4. Fine Tuning Descriptions. An editor will notice when you are either describing things too much or not enough. This allows you a second chance to fix descriptions that are too long/too short, boring, or even descriptions that are confusing because you chose the wrong word.
Eg. Describing a young woman as "homely" might sound good (and rhyme with a different word in your poetry), but the word homely actually means ugly. An editor will notice this and can even suggest words to replace it with (preferably words that rhyme).
#5. A Fresh Pair of Eyes. The biggest mistakes authors/poets mistake with their writing or poetry is that they fail to notice something OBVIOUS that they should have noticed themselves, but simply overlooked because they've read their own writing over and over again and didn't notice there are two copies of the in a sentence. Having 'the the' or similar mistakes won't be caught by spell check and may not even be noticed by grammar check. An editor will spot these mistakes and remove the extra word, spot typos and all manner of mistakes that the original author won't notice because they've stared at the words for too long they don't notice the mistakes any more.
In summary hiring a professional book editor is important. Very important. Especially if you are publishing your work, either the traditional publishing route or the self-publishing route. You want publishers and readers to take you seriously, and you want to get those positive reviews.
With respect to reviews bad spelling alone could make the difference between a 5 star review and a 4 star review, but depending upon the reviewer they might just choose a lower star value. It wouldn't be unusual for writers to get 1 star book reviews because the reviewer chose to be extra harsh.
