There are 4 main use cases:

1. Within an existing company (new product development, new business unit).  The business plan tend to focus on market potential, R&D issues, business set-up and financial requirements.
2. For a new company (start-up): the business plan provides a roadmap for the new company, its founders, its management team, its investors.
3. In a transaction context (M&A: buy or sell an existing business): the business plan provides a plan for the company pre and post-acquistion; it shows the value creation from the acquisition, provide a range for the transaction price, and might also include an integration plan.
4. Value selling for complex solutions in a B2B context: the plan shows a cost / benefit analysis of alternative solutions, and how to migrate from old tech to new tech.