You just woke up from a dream that was so incredible i.e. has imprinted itself on your mind for all time. It is the work of genius and would make a compelling story for any and all to read. The characters were detailed and the scenery was epic. The storyline was complex and intriguing making it the perfect idea for your book. You know it will be on the top of the best seller list within a few days of its release and you want to make sure you have the best representation possible.
You write down everything quickly noting every detail you can recall and set to work writing the query of your life. You have the perfect hook, just the right amount of teaser information and a close that would leave anyone wanting more. With a kiss for god luck you send off your letter to agents around the world knowing that you have just took a huge step to being a best selling author.
Within just a matter of a few weeks you begin receiving responses and great news, there are several agents that are interested! They found your idea convincing and intriguing and cannot wait to read the book. You dance around your kitchen celebrating your success and waving the letters around in the air. This is the current best day of your life. Your spouse / parent / roommate / best friend comes in and sees your celebration so that inquire what happened. You gleefully thrust the letters into their hands continuing your dance. They read a few and brighten realizing that you have multiple agents interested in you. They smile and ask "When did you finish the book?"
Slowing your dance slightly you mention that you haven't actually written it yet but now that you have someone that wants it you can have it done soon. It is pointed out that the agents have requested reading it as soon as possible and you simply quip that you are sure they will wait. They won't. I hate to burst the bubble but when you send your query the book needs to be ready to read. If an agent requests pages, chapters or the entire manuscript they mean that they want it back within a few days and it needs to be edited and prepared for them to scan immediately. They do not want anything unfinished or unpolished.
They get queries daily and will not wait for you. They will however probably remember the lack of professionalism if you make them wait or send them something that isn't up to par. No matter how great the idea is you will always be better off taking the time to do it right instead of trying to get ahead of the game before you are ready. No one can tell your story the way you can so take the time to do it correctly and give the agents and your book the respect they both deserve.