on Writing Forward:
By the time you’ve finished your manuscript and are ready to show it to agents and editors — or self-publish and put it in front of readers — you have almost certainly lost the ability to view your own work with a critical eye. While there are some tactics you can use to self-edit, you are probably too close to your manuscript to examine it objectively and see all of its weaknesses and flaws. And if you can’t see the weaknesses and flaws, you can’t fix them.
That’s where beta readers come in.
Beta readers are a group of people who read a draft of your manuscript and provide critical feedback that is intended to help you improve your work before you present it to readers, agents, or editors. Beta readers can help with a number of problems that might exist in your manuscript.