Well, in order to answer that question, I should probably attempt to explain what a beta reader is. Betas are editors for fictional stories, particularly fan fiction, although the term's usage has spread to the novelist realm. The word "beta" was borrowed from the software industry, where it is used to signify when new products are put through their paces to find and eliminate any problems with new products. That testing time enables the software creators, and nowadays the authors, to know what needs to be changed or improved.
Betas agree to fill a very important and necessary role. They review a story for any grammar, spelling, and punctuation errors. In addition, the beta may also make suggestions regarding plot continuity, characterizations, narration, word choice, word meaning, sentence construction, logical thoughts, verb tense, point of view, pacing, and canon. Of course, some writers do not desire that level of review, nor do all betas wish to expend so much time and effort. It really depends on how sophisticated the writer is: the level of technical mastery, storyteller craft, and complexity of the work.
After spending quite a bit of time and effort betaing a story, I have become rather discouraged at times with writers who have chosen to not apply my suggested edits. A writer should not ask a beta to spend time and effort on a story if he/she does not intend to take all suggestions seriously.
At the very least, a writer should present a polished story to everyone. That means the story must be as error-free as possible regarding punctuation, spelling, and grammar. A beta can help a writer reach such a goal. As for the more complex areas such as point of view, continuity, and adherence to established canon, just to name a few areas, that is up to the writer, although certainly a writer might at least review and think about all comments received by the beta relating to those areas.
That is not to claim that all comments submitted by a beta are correct. Betas are as fallible as the writers. Finding good betas is extremely important. Once you find one or more betas who know what they are doing, treat them well! Candy, flowers, effusive thanks—let them know how much you appreciate them.
Over the years, I have used the services of many betas. It became apparent to me rather quickly that different betas have different specialties. Some are grammar or punctuation experts. Others are very well-versed in particular fandoms. And then there are those who can see the overall arc of a story and provide invaluable feedback on plot direction, continuity, and characterizations.
Currently, I use the services of about four betas on each piece of writing I create. They all review the writings differently, reacting to them in unique, helpful ways. I am extremely grateful to have found them. Similarly, I provide beta services for about a half dozen writers. For some I will provide technical support, while other writers ask me for help regarding the direction of a story. Regardless of the betaing services requested, I always focus on helping the writer showcase the story in the best way possible.
Every writer should use a beta before posting or publishing a story. I cannot tell you how many times I have been thrown out of a story due to inconsistencies, misspellings, or word choice errors. Nor is it easy to continue reading a story when a writer has a character say things that are clearly out of character. A beta can catch common errors and provide guidance for the storyline. Also, since most story forums have lists of people willing to provide beta services, no writer has any reason to forego such invaluable help.
And remember to thank your beta at the beginning of your story when you do post the new and improved version. Your kind words are the only payment a beta receives.