All too often my reading is interrupted by a near miss. An author has a word in mind that might be exactly the right one, but doesn't quite nail it when it comes to typing it into the manuscript.
As an example, a mother is speaking of the beautiful view of her son's girlfriend, only the author uses the word 'site' instead of 'sight'. Or, speaking of a woman's head leaning to one side, another author wrote that it 'lulled', when she should have written 'lolled'.
No normal person could be expected to know every word. That's why it's so important to have a good proofreader with long experience to check your texts for you. Perhaps 90 percent of your readers won't know the difference. But, for the 10 per cent who do, an incorrect word smacks of carelessness. However wonderful the story may be, the moment that mistake pops up, it's not much different than tossing a boulder into the reader's path, or posting a stop sign.
A reader who knows the difference will have a moment of thinking "What just happened there? What was that supposed to be?" And their momentum through the story has screeched to a halt.
You may think you are not important enough to hire an editor, or that with enough beta readers you'll catch any problems. There are very few books that make it to the public in a perfect state. Accidents still may happen. But as a serious author, you owe it to yourself, your work, and especially, to your readers, to try to make sure your book is as good as it can be.