Beta readers are the 'beta testers' of fanfic - the guinea pigs authors try out their stuff on before posting their text to the whole audience.

Basically, the beta gives feedback of various sorts, from reporting typoes and grammar errors etc to checking the consistency of the plot, giving suggestions etc. This way the author can improve the text and make it more reader-friendly and generally better.

Also, having a beta makes the writing process less "lonely" - some use their betareaders for brainstorming as well. Being able to discuss your ideas and thoughts with someone tends to make it easier to really figure out what you want to say and how you want to say it.

Using a beta (or several!) is warmly recommendable to improve your text - for example, if you're a Finn with no clue about American slang, having a beta reader that has that cultural background can be extremely helpful. Your writing gains more credibility and texture, and even though your text was very good from the beginning, there is most probably always something that can be done even better.

A big issue, when using a beta, of course, is the fact that taking the feedback isn't always easy - but learning how to swallow some constructive criticism and benefit from it is a valuable lesson, both in writing and life in general. Although sometimes it can be hard to not be offended, one has to remember, that the beta is doing this for you. Though truth sometimes tends to be painful, better an honest beta than a brown-noser who won't tell you what s/he really thinks. How else are you going to be a better writer?

A person with an ability of giving constructive criticism is worth gold to anyone who knows how to gain from that. Having a beta lets you know what you're doing right and what you might want to do a bit differently with your fic, and can ask questions that will allow you to clarify things that might not quite make sense in the text.

Sometimes having a beta is outright fun, if your chemistries match - of course, finding the perfect beta(s) for your writing style isn't always easy. So if you find a good one, better treat him/her right!

If anyone else has something to add about being/using a beta, the benefits of that, experiences etc you might wanna share, please do!

Personally, I've had some very wonderful persons to co-operate with when writing my fic. It's great when one can trust someone enough to allow him/her to methodically go through my text to find the flaws and then help me out to fix those!

-Frost