While I still recommend the business planning process, I caution you to realize that a beautifully crafted document does not always equal business success. I've worked with many entrepreneurs who successfully launched without a plan, and some with beautifully written plans that never materialized. You and your business idea are unique. Your planning process will be unique as well. Be wary of one_size_fits_all advice or pronouncements from experts about how you should proceed.
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.