Planning is very important in life. Everybody should do planning at individual or family level. As an individual, you should plan for the day well in advance in order to accomplish what is to be done in that day. At family level, a family plan helps to focus on supporting the entire family in its growth and development. Planning is also done at business and organizational level.
When you, and your partners if any, have all the core skills and industry knowledge you need to start right away without seeking experts, napkin notes may be enough to get going. Let's say you are already an expert in technology and social media. Then you, and your team, probably don't need a detailed plan to start developing a new app. You will draw on your knowledge and experience, and you understand that you might need to go back and do some more detailed and formal planning later. Certainly when you reach the point where you are looking for investors or lenders, you will move beyond those first casual notes. Until then, drawing upon your expertise can allow you to quickly jump into the market and perhaps gain a competitive edge by using a minimalist plan.