Even if you don't have THE big idea for a business right now, it might be fun to create a business plan for a fantasy business. Whether you have a concrete idea or just want to see how a business plan works, be as specific as you can when you answer each question. If you want to borrow money to get started from friends and family, you should have your plan ready to show them.
Every successful company has a business plan. Most of them are revised and updated regularly.