It becomes easy to communicate your ideas to the people you interface with, who in turn can give you better ideas or complement the ones you have. Collect all the material to one place. Having a business plan makes it possible to have all your ideas and plans in one place. It becomes easier for you refer to it and to use it to develop your business. You become a more committed business person and even others people, like your prospective financiers, can easily regard you as a serious business person.
Remember the proverbial expression 'not seeing the wood for the trees'? You need to see the 'wood' first, then delve in and start examining the individual 'trees', meaning the individual items which you will be breaking down later. So a great point is to make sure that you have that overarching vision _ and if you cannot find one, then maybe it is an indication that you are obsessing on a few technical aspects that do not necessarily make up a whole business as you had imagined it. A business that makes sense and is going to be sustainable in the future is one that has that clear vision within which all the smaller parts contribute to make it successful.