What is the one thing that authors cannot live without?
In the end, when it really comes down to it, that is the only thing an author must have to create a success. If you don't have a computer, you can write it on paper. If you don't have paper, you use oral tradition (stretching the metaphor here). If you don't have a traditional publishing house or agent, you can self-publish. You can even print via Office Depot and sell/give away your stories, but if no one will read them, it doesn't make a difference what you do.
That is reason Number One 007 for catering to and making use of your readers.
Because of this, many authors preface their final-stage of writing with what they call 'Beta Readers'. These time-sacrificing, loyal friends/family members/significant-others can also be known as 'proofers' or 'rabid fans'. And they are your editors before you get to the editing process.
Now, there are two ways to use Beta Readers. They can read your book once you've finished the first draft. Or they can get a new chapter every time you complete one. We'll call them Final-Readers and Serialized-Readers for clarification.
Final-Readers should always be considered before you get to the agent/publishing stage. Even if you edit your book to death, employing all the different tricks and techniques to try and catch every error or erroneous detail, you will never find everything. FR can not only proof your text for errors you didn't find, but they can also give you a wide, fresh-eyed overview of your book. Did this event make sense? Was this character flaky? Do you need to change this sequence of events to a more understandable order? Was it enjoyable? FR are your fans before your book hits the shelves, and you will never find a better form of feedback and advice than in your FR.
Serialized-Readers are not necessary, but I find them to be extremely helpful and would highly recommend them to any author. Before you finish your book and do your final-edits / get to the FR stage, SR can help you expand a work in progress. Not only can they catch problems as you go, they can also be the fire under your butt to keep working. Nothing quite motivates the completion of a next chapter than hungry eyes staring at you over the computer screen, and when the fan demands, the author provides. SR are also invaluable when it comes to catching and correcting mistakes that, if given free rein until the book is finished, could cause some serious overhauling and editorial setbacks. They can also be your cheerleader as you continue, encouraging you that, yes, your book is worth writing and dangit you'd better give me the next chapter or so help me.
Both FR and SR are recommended during the writing process. Who hasn't picked up a book by their favorite author and wished they could pick their brain after finishing the read? Why didn't they make this decision? If only they'd avoided that cliche. The list goes on. You could be that for one of your writer friends, or you could avoid many author mistakes by letting/asking your friends to read your work. And unless you are the ultimate close-to-the-chest writer, there is nothing more enjoyable than having someone read your work.
The only con? Be selective. Don't give your work-in-progress to everyone you know. Don't even give it to every one of your family members and close friends. Pick a maximum of three people. I use two Serial-Readers and one Final-Reader, and that seems to balance pretty well. You can't (and shouldn't try to) please everyone. There are going to be people out there who won't like a decision you make in your writing, or even your style of writing, and you shouldn't get caught in an editorial war.
It takes time and it takes work on their part, but if you can find some Beta Readers, do so. They are the greatest asset to a work in progress beyond an actual agent who wants to throw money at you.