Usually, when an author finishes a full draft of a narrative work that they expect to publish, they will have one or more people—just regular people, not necessarily editors or publishers—read the manuscript and give him or her feedback on what they liked and didn’t like, what worked for them and what didn’t, if anything needs to be added or removed or rearranged, or any other changes the author might make to improve the manuscript, just from the point of view of an average reader. These people are known as beta readers.
In most cases, beta readers work for free; they are the author’s friends, family, acquaintances, or volunteers recruited through their publishing company. However, these people also have busy lives with other priorities, and sometimes they might be unable to give you the feedback you requested, or give you enough detail for the feedback to be useful. Sometimes it’s difficult for them to be honest with their critique due to not wanting to seem impolite or too negative. And sometimes, especially if your beta readers are volunteers you don’t particularly know or are close to, there might be confidentiality issues.
So, while looking for unpaid beta readers is always a good first step (and encouraged!), there can be many benefits to paying for a beta reading—not the least of which is an assurance of the reliability and usefulness of the feedback, and confidence in them keeping that feedback 100% private. It also helps that beta readers like me, who have worked in the industry, have the advantage of having read manuscripts of all types, so we have a clearer idea of what works and doesn’t work, how to improve it, and what next steps it might need to make it better, particularly when it comes to the editing process.