You may not have thought much about your competition or outsourcing some of your work, but things like that will impact your ability to make a profit. And you will find this especially so in the beginning phases of your business. Even you are just opening a lemonade stand in the front yard, you will still need to know what Susie is selling her lemonade for on the next street over!
I also said that a business plan consists of a narrative and several financial worksheets.
The invention process involves disclosing your invention to a wide variety of readers. As mentioned, such document is a starting point or template for providing future material with respect to the many different readers and audience for which you will need to communicate your invention.
No matter how great your idea is, most people expect that you have it written down. When I was new to inventing, I had no idea what that meant. I searched around but didn't find any universal format for documenting my ideas.