I've got another writing-related reader question this week: How do you find outside readers?
Whether or not you have outside readers, like a beta reader, critique partner or critique group, is an individual decision. Some people work better with someone else to bounce things off of while others work better alone.
There are different levels of partnership. I know of writers who get together to brainstorm plots but who then work independently. I know of writers who send each other each chapter as they go. There are those who just want a critique or review after the book is done, and they may want either big-picture notes or a detailed edit. It may be a reciprocal arrangement, in which you review each other's work, or it may be one-way if the other person isn't a writer.
Why would you want a reader? Sometimes it helps to get an outside opinion. You know all the stuff that goes behind your story, which makes it hard to tell if you've explained it well enough for someone who doesn't have all that information to understand it. An outside eye may help you find plot holes or inconsistencies. It's easy to get into the habit of reading what you think should be there rather than what's really on the page, and spotting deviations may require someone who doesn't know what should be there.
When you look for an outside reader, you're not looking for validation. It doesn't do you any good if they just tell you how awesome you are. If you need that, send it to your mom or someone else who isn't critical of you. What you need is someone who can spot the flaws before you send your work to someone who matters, like an agent or editor. You need someone who knows something about the genre you're writing in, who has experience either as a writer or as a reader. If you're developing a partnership where you also read that person's work, make sure it's a genre you enjoy reading and know something about, and make sure you like that person's work, in general.
Finding a trusted partner can be a real trick (and that's the main reason I don't have a regular critique partner). You may find critiquers in writing forums or organizations. Some writing groups provide matches or critique services. If you go to a workshop or conference, look for the person who asks the good questions or the questions you'd have asked. I know of published authors who've enlisted those readers who e-mail them with detailed critiques of their published books -- if they can spot the problems after the book is published, why not let them look at the books before they're published?
To make it work, you need to be able to trust your critique partner -- not just their opinion about your work, but not to spread it to others or post it online (that has happened). I've heard authors say it's like a marriage, so it's worth it to take some time to find the right fit.